In maths you can define functions containing variables, e.g.
f(x) = 2x+3
A common way to visualize functions like this is in a 2-dimensional graph, using the x axis to graph values of the independent variable x, and the y axis to show the computed value, f(x)
i.e. y = f(x) = 2x + 3
As muneepenee says, you can now just substitute the value 3 for x, and work out y:
y = 2 (3) + 3 = 9