It's shows the rate of change of acceleration.
If you think about a = F/m then a varying force will produce this. This happens with a pendulum, or mass on a spring, where the force depends on the displacement of the mass from its equilibrium position. So the acceleration changes with time as the mass oscillates, being greatest at maximum displacement.
You also get non uniform acceleration if the mass changes. Others have mentioned rockets. As fuel is burned, the mass gets smaller. This would cause the acceleration to get larger for the same thrust.