The graph as shown in figure below describes the motion of a ball rebounding from a horizontal surface being released from a point above the surface. Assume that the ball collides each time with the floor in elastically. The quantity represented on the y-axis is the ball's (take upward direction as positive)

We know that for a body thrown up, its displacement is given as S=ut−12gt2 . So the s-t graph is parabolic downwards. Also the ball collides in elastically, so it will rebound to less height every time as shown in the graph. t1,t2,t3 are the instants when the ball collides with ground. Here slope of the s-t graph is suddenly changing from negative to positive. It means velocity before collision is negative (downwards) and after collision is positive (upwards).