Convection currents is related to plate tectonics because it explains the reason why plate tectonics move.
For instance some geologists believe that the main factor for the motion of plate tectonics is convection currents that occurs in the mantle. This is because the mantle is made of semi-molten rock called magma. In addition just as convection currents occurs in a boiling pan this same process occurs in the mantle.
Moreover since the mantle is some kind of solid-liquid (it’s very dense) this allows for plate tectonics to float on it like oil floats on water.
In addition convection currents occurs because the very hot material at the deepest part of the mantle rises then cools sinking again and heating rising and repeating the cycle over and over again.
Thus all the motion caused by these actions causes plate tectonics to move.