The two tectonic plates meet at point one. One is moving under the other, shown by the two arrows on the top pointing towards each other. The denser plate (left side) is subducted. This means the denser plate begins to go under the less dense plate. The lithosphere (crust/tectonic plate) is mainly solid rock. As it begins to go into the mantle it will begin to heat up as the mantle is hotter than the surface. The solid surface becomes a molten rock, known as magma. It melts as it goes down.
A sea is great body of water, usually salty, that covers the earth broadly. It is a connected body of salt water that covers 70% of the earth.
The ocean on the other hand, is a large area of water in between continents. They are very big and they join smaller seas together. Together, the oceans are like one "ocean", since all the "oceans" are joined