It's the temperature differences in the mantle. Warmer magma near the core rises because it becomes less dense. It reaches the surface, pushes against the tectonic plates, then cools and falls back down to be replaced by new warm magma.
These happens in volcanic eruption and leads to the formation of volcanic rocks.
Convection is a mechanism of heat transfer (there are also the phenomena of conduction and radiation), produced by the displacement of mass through the fluid. Example: when heating water in a saucepan, contact with this solid surface produces circulation due to the gradient of temperatures that occur in the fluid in question, in this case the water.
