Let us meditate upon Wikipedia:Identifying reliable sources, a content guideline on Wikipedia that we all consider pretty fundamentally important to Wikipedia's mission. Finding reliable sources is a more fundamental research skill than just Wikipedia though: in academia, in journalism, on the sister projects (Wikinews, Wikiquote, Wiktionary and Wikispecies). If you want to say something is a fact, one good way of doing so beyond direct empirical observation, the scientific method (direct observation's older, wiser, more self-critical sibling), or logical deduction from a priori principles (mathematics, logic, various branches of philosophy and much else) is to appeal to the testimony of others. And not just anybody, but testimony of people who can be relied on to give you something closely approximating reality to a close enough degree.
But, as philosophers throughout the ages have pointed out, the idea that testimony is a source of knowledge often has problems. Not insuperable ones, mind, but problems nonetheless. These problems all tend to be caused by an unfortunate fact about human beings: they lie, they misrepresent, they bullshit and they often just misremember and fail in other unfortunate ways. It is for this reason that we have to critically evaluate what people say. This applies to representations of what they say too: and it is from this that we need to try and evaluate sources of information for quality.