Estimate means work out the answer roughly to give a good idea of what sort of value the true answer will be. This is to provide some check on the validity of the answer when properly calculated.
I would note that the denominator 40216 has a similar size to 36421, so that leaves two powers of 10 which gives us 10^-2 or a hundredth, which can be applied to 493025, a number close to 500000. Take two zeroes off the end of this to give 5000 (the result of dividing by 100). So the answer is roughly 5000. When I calculate the answer I can expect a value around 5000. Let's see: the answer is 4465.004855 which is of the sane order as 5000. I could simply have removed 25 from 493025 giving me 4930, an even closer estimate.
