Question

Definition of "plate tectonics"

Answer 1

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

Answer 2

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.