A function is a machine with exactly one job: take an input, follow a fixed rule, hand back one output. Pick a machine, drop a value in, and step through the rule one operation at a time — including the two inputs that jam the machine completely.
The machine
Every run leaves a point
Pick a machinethen feed it an input
f(3) is not multiplication. These parentheses do not mean "times" — they mean evaluated at. f(3) is a single number: the one the machine named f hands back when 3 goes in. The letter x is only a placeholder marking where the input lands.