Huge slabs of continental or oceanic crust fitting together along
their edges
what is this called?

 The border between two tectonic plates is called a boundary. All the tectonic plates are constantly moving — very slowly — around the planet, but in many different directions.

Explanation:  Plates interact at three types of plate boundaries: divergent, convergent and transform. Most of the Earth's geologic activity takes place at plate boundaries. At a divergent boundary, volcanic activity produces a mid ocean ridge and small earthquakes.