Functional application is the act of applying arguments to a [[function]]. In programming this is usually distinct from simply calling a function.
In a [[Lisp]], usually this means taking an actual list of arguments and calling a function with it:
(apply 'car '((1 2 3)))
Compare that with:
(car '(1 2 3))
In the first instance, the outer list is supplying the list of arguments to car.
In [[Lisp]], functional application is the cornerstone of a Lisp interpreter, where often in the source code a function’s arguments are quite literally applied to it.
Or, consider the following in [[JavaScript]]:
const add = (x, y, z) => x + y + z;
console.log(add.apply(null, [1, 2, 3]));