Hello. In my mind I see two different scenarios here. "For us" means, for example, person A is explaining something to person B for persons C and D. "To us" means person A is explaining it to persons C and D. Persons C and D requested that he (person A) do this. Does that make any sense?
Yes, yes. I hadn't thought of this. Yeah, what kayokid and JamesM are saying about "for us" is definitly true. "Explain it for us" is asking someone to do your job of explaining for you. "Explain it to us" is asking someone to help you to understand something.