The video shows a man transporting a fish from a bucket of nice fish-friendly water to a container filled with an unspecified substance (we know it's not liquid nitrogen, as the temperature is shown at -35 in the beginning, which is too warm for nitrogen to still be in liquid form, no matter the unit).
A subduction zone is formed at a convergent plate boundary when one or both of the tectonic plates is composed of oceanic crust. The denser plate, made of oceanic crust, is subducted underneath the less dense plate, which can be either continental or oceanic crust.
Convergent boundaries are plate boundaries formed when two tectonic plates crush together. The denser of the two plates goes under the other! For example, if two oceanic plates (which are tectonic plates found in the ocean) collide, the denser of the two plates goes under the other and forms an oceanic trench. If an oceanic plate and a continental plate collide (a continental plate is a tectonic plate that holds land/continents), the oceanic plate will go under the continental plate and goes into the mantle. If two continental plates collide, the crust thickens and is forced to create landforms such as mountains.