Explain me the meaning of the sentence-'the mantle rises beneath the Earth's crust before it spreads sideways, and then cools again.'

Deep beneath the surface of the earth, heat rises from the core, goes through the mantle and reaches the crust. It comes up slowly but causes the mantle to move. The mantle rises beneath to Earth's crust before it spreads sideways,and then cools again. This slow but constant movement of the mantle divides the Earth's crust into plates called tectonic plates.