In math, a function is a relationship between two sets where each input from the first set is assigned to a unique output in the second set. This means that for every input, there is only one output.
The table shows a function because for each day of the week (input), there is only one corresponding number of children in the car (output). For example, on Monday, there were 4 children in the car, and this value does not appear on any other day of the week. This satisfies the definition of a function where each input has a unique output.