In this case reserves meant something like “the stuff that doesn’t automatically replenish within a week.” In the case of money, this’d literally be “the amount of money you make in a week minus living expenses.” Sometimes you make bigger purchases, but you should reflect before you do.
It’s a bit subtler/weirder for non-money things. There is some rate at which you (or, at least, I) can call in favors, or pull all-nighters, or deal with intense social drama, before it starts affect my ability to accomplish my usual day-to-day-activities. For example, if stay up late cramming to finish one assignment for work, I can still go into work the next day and do my job. If I did that for a week, I’d become less effective.
What counts as “reserves” depends both on how many recurring obligations you have, and how much resources you have.