What does acceleration mean in motions graphs?

is defined as the rate of change of velocity.

This means that if you plot velocity (y axis) against time (x axis) for any object, the GRADIENT of the graph is the acceleration.

Some useful features you can identify:

If the gradient of the velocity-time graph is constant (i.e. it is a straight line) you can identify that the acceleration is constant. This is useful as it allows you to apply SUVAT equations to find unknowns.

If the gradient of the velocity-time graph is positive, this means that the velocity is increasing or if it is negative that the velocity is decreasing.