Subduction is from the Latin, “sub” meaning “under” and “ducere”, “to pull”. It is a limit between plates where the lithospheric plates are converging, i.e. approaching each other, with one plate (generally oceanic) sliding under the other plate (oceanic or continental).
Because the size of the Earth is constant, the crust formed at the ocean ridges must of necessity go somewhere else.
This happens at the convergence of plates (oceanic or continental). Along the line of convergence, the denser plate passes under the other one. This is called subduction.
Subduction is usually accompanied by partial melting of the descending crust or of the surrounding mantle, which can give rise to volcanism behind the subduction zone.