Here is the origin of the expression:
(quoting a previous post of mine I found on my PC)
The complete version of the expression is: to make both ends meet.
Here is the most interesting explanation for me:
The expression originates from bookkeeping (accounting), where you have two columns of numbers, one for income and one for expenditure;
the totals at the bottom of the columns are called the "ends";
the 'end' of income column should (at least) match (meet) the 'end' of the expenditure column, for things to go smoothly.
If the two values at the end meet (are the same) of course there is nothing more to spend, as has been already pointed out.