Hello!
A function is a system in math where you plug in an input, and that function will produce one output from the input. In a function, the input is usually represented by x, and the output is usually represented by a variable of x, most commonly f(x)
For example, we have the function f(x)= x+15. You may be asked to find f(x) when x=14. We would plug in our value into the function.
f(14)= 14+15
f(14)= 29
We would say this as the f of fourteen is 19.
I hope this helps!