There is a constant leitmotif in philosophy, one of a struggle between skepticism and certainty of belief. The evolution of 21st century epistemology from the episteme of the Ancient Greeks has been exemplified by a metaphorical Sturm und Drang of thinkers who wage contentious campaigns to make certain claims (think Descartes) or declare the act of claiming itself pointless (think Nietzsche). One particular period in history that reflects a drive towards epistemological certainty was that of the logical positivists, who armed with the formal sciences, set out to show that the ambiguity that inheres in scientific practice and theory could be carved out rendering epistemological science as reliable as an apparatus of Newtonian mechanics. They did so by declaring war on metaphysics and tried to show using the gains in mathematical and logical theory during the end of the 19th and start of the 20th century, that science could be reduced or explained formally with Ramsey sentences, deductive-nomological reasoning, and other clever arguments. But unfortunately, they bumped into a series of challenges that didn't readily dissolve. Carnap for instance hewed mightily to belief in the the analytic-synthetic dichotomy grasping at the disembodied and transcendent logic that Kant himself argued for in his Introduction to Logic.
Enter a series of developments and personae dramatis that doomed the project so that the engineers themselves conceded defeat. Karl Popper took confirmation and verification to task. Quine attacked the analytic-synthetic distinction. Kuhn argued that scientific theories were sociological artifacts with political dimensions. By the 1960's logical positivism was well past moribund. Theory-ladenness and underdetermination of theory helped to bring the coup de grace. Let's analogize.
Let points be our facts of a theory, and let various claims regarding the relationships, the building blocks of a theory, be represented by curves that connect the dots. In any locus, one can pick three points, and draw a curve. In fact, we say that three points underdetermine a curve, because given three points, there are an infinite number of curves that connect them. A theory, then in our analogy, is putting all of the curves together in such a way to have a polynomial. This mathematical practice is less known that linear regression, and is aptly called polynomial regression. So the methods provides that the same points can determine different polynomials, and unfortunate there is no objective property inherent in the points that determines the polynomial, unlike 2 points determining a line, 3 a circle, etc. There is no one true polynomial.
Thus if one asks, which polynomial is represented by these points, and one has a method that says all of these polynomials fits, then in essence, one really hasn't determined anything. Thus the more points the method of regression fits, the more polynomials are feasible, and the less that is actually said about which one polynomial. An imperfect analogy, certainly.
So, likewise, when scientific facts determine more and more theories under the "scientific method", the less and less the scientific theory that combines various theories says. According to SEP, this is the underdetermination of theory by data. Or in other words, a theory that explains everything explains nothing (a clever dialetheism with all the hallmarks of a koan). For opponents of scientific realism, this is evidence there is no one true reality, and theories are merely instruments selected from preference and bias.