Estimating your expenses means checking of the money you have spent on buying goods or services over a period of time.
On individual basis, a person can estimate his monthly expenses by writing down about where he spent his money. He can spend money on many things like house rent, electricity bill, phone bill, credit card bill, grocery, tuition fee, and on many other things. These are called the expenses he has made over a month.
Similarly companies also keep record of their expenses. There expenses are big and different as compared to individual household expenses. So all such things are estimating the expenses.