There are many examples to choose from, but one of them is the idea of distance, speed, and time being linked together.
Let's say a car is going 60 miles per hour and it does not change its speed. If x is the time in hours, and y is the distance traveled, then we have the equation
y = 60x
which is due to the d = rt equation (distance = rate*time)
As x goes up, so does y. If x goes down, then y does too. The two variables move in the same direction together. This is one way to think of what it means to have a direct proportional relationship here. In contrast, an inverse proportional relationship would have x going down to make y go up, and vice versa.
Side note: the equation y = 60x is in the form y = kx with the constant of variation k = 60.
