Estimation is making an educated guess as to the amount or size of something, without actually performing the calculation or measurement required to obtain the exact value. Estimation is important in day-to-day use of mathematics in situations where coming up with an answer quickly is more important than precision. For example, say that you're at a grocery store and you have $40 in cash and 17 items in your cart. It's important for you to know whether the total is less than $40 before you check out, but it probably doesn't matter whether the exact total is $36.02 or $36.87. Estimation is an important part of number sense.
You may also want to try these estimation exercises: