What is truth? Truth is a consensus. We can only judge reality through our perceptions, and those perceptions are demonstrably fallible. So truth is only what we can agree on. In the scientific method, evidence is paramount in demonstrating truth. The scientific method is the best mechanism we have for finding truth. It is tempting thus to implicitly accept all information as truth from persons who refer to themselves as scientists. This leads to an argument from authority fallacy. It is possible, after all, for someone who refers to themselves as a scientist to not practice the scientific method. Therefore, we introduce credibility to replace authority as a metric for evaluating a claim. Credibility is determined by examining all past claims of this person and judging whether or not the provided evidence supports their claim.
Credibility is domain specific. Judging a claim includes both judging the individual as being fundamentally intellectually honest and a steward of truth as opposed to one who is subject to biases or who makes exceptions to truth seeking in special cases, and also judging whether the credibility domain of the individual includes the claim they are making.
It is possible for even credible sources to be wrong though. So we cannot just implicitly accept the claims of credible sources, they still must be evaluated ourselves. Using credibility, we can establish an order in which we evaluate claims, since we have no time to evaluate them all. In an instance where two claims are equivalently supported but contradict each other, we will go with the claim of the more credible source until the less credible provides more evidence to overcome the credibility gap.
Information overload is the big problem of this age. We need efficiency in sorting through the barrage of information that bombards us from all angles.Determining a source's overall credibility as well as his domain credibility is the most important thing we can do to ensure that we are not deceived. Weeding out junk claims in this fashion will force people to provide more solid evidence and reasoning to gain our attention, which means less of our time is spent entertaining claims which lack truth.