What does it mean if two variables are in direct proportion?

Two variables are in direct proportion if an increase in one variable causes an increase in the other. This can be something simple like f(x) = 5x. Doubling x will cause f(x) to double. The proportionality doesn't have to be linear. It can be quadratic, cubed, or logarithmic. As long as an increase in one causes the same in the other, direct proportionality holds true.