What is estimation?

Estimation: The estimation of a number is a reasonable guess of the actual value. Estimation means approximating a quantity to the accuracy required. This is done by rounding off the numbers involved and getting a quick and rough answer. 

Estimation of the product:

To estimate the product, round off each factor to its greatest place, then multiply the rounded off factors.

For example,

Estimate 92 $\times$ 57

92 can be rounded off to the nearest ten as 90.

57 can be rounded off to the nearest ten as 60.

Hence, the estimated product $= 90 \times 60 = 5400$.