This lesson is all about justify your answer by using "estimation."  An estimate tells you "about how much", it is not the exact amount.

The idea behind estimation is to use numbers "close to" the actual numbers in the problem so that you can do the calculations "mentally,"  and "quickly".

You can estimate to:
justify if your answer is correct
check to see if your answer makes sense

Estimating to find a number close to an exact answer is one way to justify your answer. Then complete computation and compare results to justify your estimation.
 

Student council sold tickets to the school play which would run 3 different showings. Together they sold  414, 567, and 308 tickets. They needed to sell at least 1000 tickets to earn enough money to cover the expenses. 
Did they do it?

Since you only need to know if the sum is more or less than 1000, we can estimate the answer.  The sum of 400 + 500 + 300 = 1200.  Since the estimation we used for each number is less than the actual number, and since the sum of those estimates is greater than the target number of 1000, the actual sum must be greater than 1000.
They did it!

Now let's add the actual numbers and compare it to the estimation.

414 + 567 +308= 1289

As you can see, estimation can be used to justify the reasonableness of your answer!