The plate tectonic theory is about the movement of large sections of Earth’s crsut.
The plate tectonic theory talks about how the crust move apart, converge/collide in one another and slide part each other on its boundaries.
It also talk about the features formed(e.g. volcanoes, fold mountains) on the boundaries, some natural disaster such as earthquake and tsunami can also be explained through it. It also talks about how the plates collide/move apart/slide part each other.
For example, destructive plate boundary is where the crust collide due to sinking convection current of magma. Denser oceanic plate will subduct under the less dense continential plate,forming volcanoes and ocean trench.
Hope it helps!