Basically f(x)=5x means whatever number you plug in will be your x coordinate, and the result will be your y coordinate.
(1,5) (2,10) (3,15) (4,20) and so on. 1,2,3 and 4 are x coordinates plugged into the equation, the y coordinate is whatever 1,2,3 and 4 multiplied by 5 is.