The average of the values in the range A1:A100 is calculated.
When you have something nested, in this case function calls, but it also can take place in computer programming, you have the result of one function call become the argument of a separate function. AVERAGE(A1:A100) will run first. Let's say the result is 83.5 Then INT(83.5) will run, resulting in an 83 as the result in the cell the nested function exists in.