A Fermi Problem, named after Enrico Fermi, is a question about quantity or order of magnitude, needing approximation and estimation to answer, with very little to no information.
Example Fermi Question
How many piano tuners are there in Chicago?
Solving Fermi Problems
Because of the lack of information those problems use the fact, that the average of many semi-correct estimations results in a much more accurate approximation.
In this way we estimate the partial information needed to calculate/come up with the final answer for our question.
Another trick is to round the numbers to ones that will be easier to calculate on, and use those as approximation to arrive at the answer.