Hi Marcos, and welcome to the forums!
Rusty's right on with the translation. The verb
tratarse de algo means
to be about something.
This book is about politics.
Este libro se trata de la política.
As a side note, I *think* that in other places, namely Spain, you can use tratar (non-reflexive) with the same meaning:
Este libro trata de la política.
Sosia, Alfonso, María etc correct me on this if I'm wrong.