Convection means a certain type of movement of a fluid. Convection occurs in atmosphere, the oceans, in a boiling pot of water, among others. The movement is also known as convection currents. In the case of the Earths crust, convection means that the liquid magma on which the crust floats is moving in a circular fashion, rising from higher depths closer to the crust. When it reaches the crust it changes direction and flows underneath the crust, making it move.