What does the area under the graph represent in a velocity-time graph?

Distance travelled, or change in displacement. This usually means the same thing at GCSE.

The area under a curve down to the x axis represents the distance travelled.

area under a velocity-time graph is the displacement

In a velocity-time graph, the area under the graph represents the distance an object has traveled over a period of time. If you calculate the area between the velocity line and the time axis from one point in time to another, that area will tell you how far the object has moved during that time interval. If the velocity is above the time axis (positive), the object is moving forward. If it's below the time axis (negative), the object is moving backward.