The following excerpt from Philosophical Foundations for a Christian Worldview by William Lane Craig and J.P. Moreland (p.88) is a helpful explanation of defeaters for beliefs:
“One factor that affects whether and to what degree a belief is justified is the presence of defeaters for that belief. Suppose Smith has a blief that Q (e.g., that a statue is blue), and suppose that R (e.g., the way the statue looks to him) is a reason or ground Smith has for holding to Q. A defeater removes or weakens justification for a belief.
There are at least two kinds of defeaters. First, there are rebutting defeaters, which directly attack the conclusion or thing being believed. In the case above, a rebutting defeater would be a reason to believe not-Q, i.e., a reason to believe that the statue is not blue. An example would be a case where the museum director and a number of reliable, honest people assure you that the statue is grey.
Second, there are undercutting defeaters. These defeaters do not directly attack the thing believed (by trying to show that it is false), but rather they attack the notion that R is a good reason for Q. Undercutting defeaters do not attack Q directly; they attack R and in some way undercut R as a good reason for Q. In the example above, an undercutting defeater would be evidence that there is blue lighting around the statue that makes everything in that room look blue to people.
In the example, the undercutting defeater removes one’s reason for thinking that the statue is blue and the rebutting defeater gives one reason for thinking that the statue is not-blue. In different ways, defeaters can remove the justification for a belief.”