The CAP theorem is probably the most popular theorem in computer science. At a very simplified level, failures will happen in a distributed system. When a failure does happen, the CAP theorem says you will need to choose between consistency (clients always read the most recent data) and availability (clients always get a non-error response).
It's fun to dream about building large-scale distributed systems and how you'll handle the difficult choice of availability vs. consistency. Of course you'll choose the cutting-edge database that avoids the CAP theorem entirely (and heck, maybe it solves the P vs NP problem while we're at it)!
Joking aside, I'm going to tell you a little secret -- you shouldn't really care about the CAP theorem.