Hi Ben, I don't quite understand your question.
Resource calendar shows resource availability to aid you in your schedule planning. Let's say you need someone to work on task A on day 1 to day 5 and you notice that Person A is available, then you will update the resource calendar indicating that Person A will not be available day 1 to day 5 since he will be working on task A. This will also show whether Person A is available for other projects or not (in case some other project is eyeing on the same person).
I hope the above answer your question. If not, please post your question again with an example.
Thanks!