Benchmark numbers are the two numbers a number would fall between on a
number-line (essentially, the numbers it would possibly round to).
For example:
12,432
The benchmark numbers for the ten thousands would be 10,000 and 20,000
(12,432 is found somewhere between these two numbers).
The benchmark numbers for the thousands would be 12,00 and 13,000 (12,432 is found somewhere between
these two numbers).
The benchmark numbers for the hundreds would be 12,400 and 12,500 (12,432 is found somewhere between these two numbers).
For the homework, students have to place these two numbers on a number-line and find where the actual number (12,432) would be.
We do this to help prepare for rounding - to start to visualize which benchmark number it is actually closer to.
Here is a link to a guided lesson on benchmark numbers:
http://learnzillion.com/lessons/523-locate-benchmark-numbers-on-a-number-line
I hope this helps!