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.
There's no right or wrong way of making an estimate. A relatively easy way of estimating the results of a numerical calculation is to round values to numbers that you can easily work with in your head, and then perform the calculation using those numbers. For example, if you want to estimate the value of 17 × 19 × 23, you could calculate the value of 20 × 20 × 20, giving a rough estimate of 8,000. If you're estimating lengths, areas, volumes, or angle measurements, you can compare those against known quantities.
Probably the best way of becoming a good estimator is to make estimates, and then check how well you did afterwards. Whenever you have to make a calculation or a measurement, estimate the value before calculating or measuring. You can also make estimates for questions you don't have to know the answer for. The book Guesstimation by Lawrence Weinstein and John A. Adam (read book review) provides a large number of interesting questions on which you can use your estimation skills.
You may also want to try these estimation exercises: