Autolisp is a variant of the lisp programming language. It can be used to drive autocad commands, but it is also a full-featured progamming language.
Autolisp is an interpreted language: expressions are evaluated immediately. There is no compilation process required as with C or C++.
If you type (+ 2 5) at the autocad command prompt, and press enter, you will see the value 7 returned.
The result (return value) of an autolisp expression can supply a value to autocad command prompts. For instance, if you had invoke the circle command, pick a center point, and then supplied (+ 2 5) for the radius, autocad would proceed to draw a circle with a radius of 7.