Numbers with more than two digits can be taken apart to create estimates. Some times it is easier to work with estimates rather than the exact number. For example: 42 is 40 and 2. The skills of estimating is very helpful in many ways.
Therefore, the term estimate refers to a number that is the best approximation for an exact number in a given context.
Always read your questions carefully to make sure it is asking you for an exact number or an estimation. Here are some exercises to help you develop your estimation skills.