Full Page Cache aka FPC is made to speed up your shop by creating simple file to load instead of the all opencart framework. This extension will cache all your catalog pages but will keep the user specific information dynamic.
DYNAMIC USER BLOCK
A page of the catalog is mostly static. Only few part of it change, in function of the action of the user. This part, I will call them the "user block". In the default theme there is 3 user blocks: the cart, the wishlist and the login/welcome message. Each of this user block will be identify by the FPC parser and replaced by the current user data.