SE* do not provide assurances of truth; they provide information.
First off, it is a collection of short articles -- just the written content of the answers will provide information, e.g. summaries and references to other works, individuals' experiences, etc. However, the key feature of SE beyond providing a platform for user generated content, is the voting system.
At the most bare level they provide information on the "goodness" of answers, where goodness is defined in the tautological sense: i.e. good answers are those answers that are voted up (similar to the tautological definition of fitness as propensity to reproduce sometimes used in describing evolution). There is a special vote that comes from the asker (accepted) and other votes that can come from anyone.
For questions that relate to clearly proscribed instrumental goals the interpretation of the goodness votes is pretty clear:
- The asker asks question Q in order to achieve goal G
- Answerers provide answers A1, A2, ...
- The asker accepts (and usually upvotes) the answer, An that he/she used to achieve the goal.
- Other people, with questions Q' and goals G', to come via search engines to question Q; if an answer, Am allows them to achieve their goals, they upvote it.
- (As an aside, upvotes on Q indicate that many people searched in such a way as to find that question, and found it, and the answers, useful in achieving their goals)
There is a lot of truth in this -- not that the answers are true, but rather these records of what people have done, in terms of finding the question, achieving their goals, etc., are truthfully recorded and reproduced (assuming no software glitches).
In terms of practical reasoning this is usually sufficient -- you have self-reported information on problems other people have had, and which answers were sufficient to address them. This is probably the most common way of justifying ones beliefs in day-to-day life: relying on people's past experiences. In terms of formal/logical justification of the value of the answers it is clearly insufficient. If you assume that there is a close correspondence between the voting and the behaviours outlined above, then you can believe that SE provide at least (an analog of) empirical adequacy; whether you want to count this as "truth" or something like it is a word game.
Aside on the utility of sorting by votes: If you assume that the voting population is representative of the viewing population, then sorting by votes minimizes the average depth to which people have to search in order to find a suitable answer. This is another not truth-related, but informative aspect of the sites' designs.
For less concretely defined questions, the ability to make these higher level generalizations is correspondingly weaker. For example, if the answer relies on a sense of aesthetics (this can occur in programming!) then the resulting distribution of votes will reflect the aesthetic proclivities of the voting population.
Note that the SE model (I think not coincidentally) satisfies the critera set out by J. Suroweicki in The Wisdom of Crowds (1) diversity of opinion; (2) independence of members from one another; (3) decentralization; and (4) a good method for aggregating opinions.
Most obviously, this is an elaboration on Shiri's answer, and there is some overlap in this answer and Alexander S King's, in that I tried to more fully flesh out what "objective" means in the SE context, and Cort Ammon's, in terms of trying to flesh out the circularity.
(*) I'm using "SE" to refer to the set of Stack Exchange sites.