buffer: db 'hello, world'
times 64-$+buffer db '_'
Assume the location of buffer is 1000, then the location of times (identified by the $ sign) is 1012.
So:
64 - 1012 = -948
-948 + 1000 = 52
The expression effectively calculates the number of dashes to add to the previous string to make a total of 64 characters.