That means don't use a calculator.
2 * 12.5 = 25 about or you could use 2 * 13 = 26. Now you can use a calculator to see how close you were.
1.75 * 12.5 = 21.875
You may dismayed that you are not any closer, but the idea is to get a close answer without using a calculator. You are actually looking for a range of numbers that could contain the answer.
If you want the lower part of the range you could round to 1*12.5 = 12.5
So if you get an answer like 3.8 or 400 you know you did something wrong. Learning to estimate is a gift you should take out of every math or science class. It is a give freely given.
Estimate: betwee 12.5 and 25