Estimation is the process of finding an estimate, and that is usable in many ways. The value is usable because it is guessed from the best information he/she has. For example, you want to know the number of atoms a plastic bag has. The information you have is: Atoms are very small. That means the bag must have tons of atoms. So, a pretty good guess for a starter is 10,000,000,000,000 atoms.
Another example is that you want to know how many pages a fairy tale has. Your information is: Paper is extremely thin, and the book is about a millimeter thick. So what do think the value will be? 10? Maybe. 20? Maybe. 30? Maybe.
