(Author's Note: This article is about navigating large game code bases, which can go up to and past the 2 million lines of code mark and are usually mostly C++. I'm sure these tips are applicable to other industries with comparable size projects, but you write what you know)
Freshly hired, you sit down at your desk at your hard won job at Big Game Studio. You're making games! Big ones! Excited to get started, you've gone through the orientation, pulled the source tree to your machine, and fire up the IDE. You wait for it to load... and wait... and wait... just how large is this project? Your producer has already given you a simple task for their editor -- make the FooBaz window remember its position and size between runs of the editor. You have no idea where to start. A sense of panic comes over you as you realize little in your previous experience has prepared you for dealing with a code base this big.
It is not uncommon these days to land a game programming gig and have to deal with a large legacy code base. It could be all in-house code, it could be a licensed engine, or it could incorporate a lot of open source software. Unless you are working on a small game, more than likely the first thing you are going to have to learn is how to navigate this beast.