Definition of "plate tectonics"

A theory explaining the structure of the earth's crust and many associated phenomena as resulting from the interaction of rigid lithospheric plates that move slowly over the underlying mantle.

According to a website, it describes how Earth's outer shell is divided into several plates that "glide over the mantle (which is the rocky inner layer) "

So, basically, it's talking about how the Earth's crust is structured.

Definition: a theory that is describing (or explaining) the structure of Earth's crust