The domain of a function is the set of numbers that can be used as an input. For every case when we are dealing with real world objects the domain can't have negative numbers. For example: If a function is modeling the revenue of a parking lot as a function of the number of cars that park there in a month, there is no negative car, therefore we can't have negative values as inputs. The worst case scenario would be a situation where no cars visited the parking lot for the whole month, which would be an input of 0.
