Merriam-Webster defines plate tectonics as "a theory in geology: the lithosphere of the earth is divided into a small number of plates which float on and travel independently over the mantle and much of the earth's seismic activity occurs at the boundaries of these plate".
The earth is like a giant stove and the continents are like the water in the pot on top. The things on the sides spread away and in the middle, it goes upwards, so everything else spreads around.
The Earth has trenches in the ocean and at the bottom of the trenches, new land from magma is being created, spreading the other Earth outward. In other places in trenches, one plate goes under, while the other goes up. This is called subduction.