What does a velocity vs. time graph show?
Velocity vs. time graph shows the variation of velocity with time.
If the velocity-time graph is a straight line parallel to the x axis, the object moves with constant velocity.
If the graph is a straight line (not parallel to the x axis), the velocity is increasing uniformly i.e. the body moves with constant acceleration.
The slope of the graph at any point gives the value of acceleration at that point. The steeper the curve at a point, the greater is the acceleration.