An excuse is often, in fandom, a poor reason for why something a fan has written or otherwise done is of low quality, why it is so short, why it has not been updated in forever, and so on. Poor excuses are charges. For discussion of some of the excuses most commonly encountered by the PPC (and why they cut no ice with us), see the FAQ For Other People, Section 6.
Sometimes, however, excuses are genuine, though this can be hard to determine.