An appeal to motive is a fallacy that tries to discredit a person’s argument by pointing out the person’s motive.
For instance, “Johnny’s argument is false because the only reason he’s presenting it is because he wants to make us feel guilty.”
While it usually is morally wrong to make someone feel guilty, the fact that Johnny’s motive is bad does not mean his argument is bad. One has to deal with the argument itself, rather than the person presenting it.
On the flip side, it really hurts someone’s credibility to present an argument with an underlying motive. A wrong motive is enough to get someone not to listen to your argument no matter how strong the argument is.