With myFunction(4, 5), that is the caller, which calls the function called myFunction, so the place in the code which defines the myFunction function using function myFunction is the callee.
let x = myFunction(4, 3); // caller
function myFunction(a, b) { // callee
return a * b;
}
The return value of the myFunction function is the resulting value from a * b.
That value gets used in place of the caller, which in your code ends up being assigned to the variable called x.
I recommend you read a book for beginners and go through the lessons in order because they build upon earlier lessons. Going through videos in a random fashion will always leave out some concepts
