The lithosphere isn't just one continuous covering. It is divided into sections called plates . Three things can happen. They can collide, move apart, or slide past each other. As new crust is added at divergent boundaries, it is also recycled at convergent boundaries.
Convergent boundaries are boundaries where the plates collide.
As they crash into each other, one plate almost always slides under the other plate. Where subduction occurs old crusts is moved down into the mantle where it melts. The magma is less dense which rises to the surface forming volcanoes. If you were to plot volcanoes on a map, you would see a pattern. Many of the world's volcanoes are located in a region called the Pacific Ring of Fire where the Pacific plate collides with other plates.