Because could is a form of can, the sensible differences between those requests come from tense/mood.
Can inquires about the present, definite future, or general ability/possibility:
- Can you explain it (now)?
- Can you explain it (later, if I come to your office)?
- Can you explain it (under any set of circumstances)?
Could indicates the subjunctive:
- Could you explain it to me (if I were to specify the question)?
- Could you explain it to me (we were to find twenty minutes to talk)?
- Could you explain it to me (if you weren't doing something else right now)?
If we mean to use can/could politely we're using them in pointed contrast to literal meaning: we're politely interested in convenience, preference, etc., and for that reason we do not ask about those things, and allow the conceit of ability to determine the outcome. And also asking after ability allows someone to issue a more definite "no" than asking after preference, convenience, whch is important.