As a follow up to causative 's excellent answer, which pithily gets to the gist of the matter, here is a pretty succinct outline of the overwhelmingly and timelessly prevalent traditional correspondence theory of truth, as well as its two most traditionally popular competitors:
The Correspondence Theory of Truth: The Correspondence Theory of
Truth is probably the most common and widespread way of understanding
the nature of truth and falsehood. Put quite simply, the
Correspondence Theory argues that “truth” is whatever corresponds to
reality. An idea which corresponds with reality is true while an idea
which does not correspond with reality is false. It is important to
note here that “truth” is not a property of “facts.” This may seem odd
at first, but a distinction is being made here between facts and
beliefs. A fact is some set of circumstances in the world while a
belief is an opinion about those what those facts are. A fact cannot
be either true or false, it simply is because that is the way the
world is. A belief, however, is capable of being true or false because
it may or may not accurately describe the world. Under the
Correspondence Theory of Truth, the reason why we label certain
beliefs as “true” is because they correspond to those facts about the
world. Thus, the belief that the sky is blue is a “true” belief
because of the fact that the sky is blue. Along with beliefs, we can
count statements, propositions, sentences, etc. as capable of being
true or false.
Especially after Kant’s distinction between noumena (things in themselves) and phenomena (the world of appearances; perceptions and apperceptions), in his Transcendental Idealism (https://plato.stanford.edu/entries/kant-transcendental-idealism/), and the purported failure of traditional “Realism” (tout court, or in this or that domain), it became impossible to make/have non-inferential/mediated claims/beliefs about an “objective” world (noumena/things in themselves), a so called view from nowhere. So, competing theories about the nature[lessness] of truth were developed, the most traditionally popular of which were:
The Coherence Theory of Truth: The Coherence Theory of truth is
probably second in popularity to the Correspondence Theory even though
it often seems to be an accurate description of how our conception of
truth actually works. Put simply: a belief is true when we are able to
incorporate it in an orderly and logical manner into a larger and
complex system of beliefs or, even more simply still, a belief is true
when it fits in with the set of all our other beliefs without creating
a contradiction.
The Pragmatic Theory of Truth: The Pragmatic Theoryof truth determines whether or not a belief is true or not based on whether it has a useful (pragmatic) application in the world. If it > does not, then it is not true. As with Coherence Theory, truth in this > sense is nothing to do with the way the world ‘really is’ but is just a function of whether an idea can be used as a model to make useful predictions about what is going to happen in the world. As a result pragmatic truths can only be learnt through interaction with the > world: we don’t discover truth by sitting alone in a room and thinking about it.
But particularly over the last Century+ there have developed many increasingly "deflationary" theories of [no longer capital T] truth (including Causative’s preferred theory, developed by C.S. Pierce, and elucidated further by Hillary Putnum):
Deflationary theories of Truth: Deflationism about truth, what is
often simply called “deflationism”, is really not so much a theory of
truth in the traditional sense, as it is a different, newer sort of
approach to the topic. Traditional theories of truth are part of a
philosophical debate about the nature of a supposed property of truth.
Philosophers offering such theories often make suggestions like the
following: truth consists in correspondence to the facts; truth
consists in coherence with a set of beliefs or propositions; truth is
what is acceptable in the ideal limit of inquiry. According to
deflationists, such suggestions are mistaken, and, moreover, they all
share a common mistake. The common mistake is to assume that truth has
a nature of the kind that philosophers might find out about and
develop theories of. The main idea of the deflationary approach is (a)
that all that can be significantly said about truth is exhausted by an
account of the role of the expression ‘true’ or of the concept of
truth in our talk and thought, and (b) that, by contrast with what
traditional views assume, this role is neither metaphysically
substantive nor explanatory. For example, according to deflationary
accounts, to say that ‘snow is white’ is true, or that it is true that
snow is white, is in some sense strongly equivalent to saying simply
that snow is white, and this, according to the deflationary approach,
is all that can be said significantly about the truth of ‘snow is
white’. Philosophers looking for some underlying nature of some truth
property that is attributed with the use of the expression ‘true’ are
bound to be frustrated, the deflationist says, because they are
looking for something that isn’t there.
https://plato.stanford.edu/entries/truth-deflationary/.
(Also see, inter alia for instance, Lynch’s Truth in Context, and Blackburn’s Truth a Guide
https://www.amazon.com/Truth-Context-Essay-Pluralism-Objectivity/dp/026262155X
https://www.amazon.com/Truth-Guide-Simon-Blackburn-ebook/dp/B00UNRO9HA/ref=sr_1_2?crid=2M9PURIKQ1LEQ&keywords=truth+a+guide&qid=1645386306&s=books&sprefix=truth+a+guide%2Cstripbooks%2C147&sr=1-2).
Is it any wonder that we live in a “post-truth” world amongst a plethora of competing “alternative facts,” where, “chimera” like “reality,” “the world,” “objective facts” etc. are no longer non-naively believed to constrain what can be claimed or believed? Where "knowledge" itself is up for grabs.
