Cells and the structures inside them are very small, so we compare their sizes using powers of ten. A typical bacterium is about 1 µm across and a typical animal cell is about 10 µm across. The animal cell is 10 times wider, which is one order of magnitude bigger. If something is 100 times bigger it is two orders of magnitude bigger, because 100 = 10 × 10.
An estimate is an approximate value, usually found by rounding numbers to make them simple. We use an estimate when an exact value is hard to measure, when it is not needed, or to check that a calculated answer is sensible. For example, if 20 cells fit across a field of view that is 1000 µm wide, each cell is about 1000 ÷ 20 = 50 µm wide.
Estimation is also used to count large numbers. You count the cells in one small area, then multiply up to the whole area. The result is an estimate, not an exact value, but it is quick and good enough to compare samples.