Student or Learner
What does "keep a running total" in the following sentence mean?
...I kept a running total, but didn't worry about the amount too much...
a total that is continually adjusted to take account of items as they are added.
If you are on a budget, then you need to keep track of your expenditure as you buy. So each time you buy something, you re-calculate the total you have spent so far.
A running total allows you to keep track of how much you are spending; or that the other person owes you as they buy from you or run up some other debt to you.