As to not start another 100-post topic, you can read this thread:
Although it is quite long so you may want to reserve some time for it. 
The gist: In math you can have functions like f(x) = x * 2. x would be a parameter and if we put in 2 for x we would get 4. That’s what the return block does, except you can return anything from a function, including sprites and strings and arrays and whatever! In the functions list there will be a circle version of any function with a return <value> block in it.