That's a good question, because they never tell you exactly how it works, and different variants work in different ways.
The basic gist of it is that you can install the game 5 times in total on different machines or different OS installs. On the same machine you're supposed to be able to install and uninstall as many times as you like. However they never tell you what a 'machine' means - a different OS on the same machine can be considered by the DRM to be a different PC, or even a different account can be considered a different PC. If you change major hardware like a CPU or Motherboard the DRM may burn through another activation. But you'll never know until it happens, or in extreme cases you won't know until your game suddenly refuses to activate.
Most new versions of Securom will 'revoke' an install, giving you back an activation when you uninstall the game, though I've read that Crysis does not do this, and there is not separate revoke tool either!
Sometimes the activation system can go wrong and your game won't activate, or you run out of activations, though this is rare.
To their credit EA are very good about giving extra activations via their live chat online support system.
Only some of EA's games let you know how many activations you have left. Most of the time you're left guessing.
I really despise limited activation DRM and will do my best to avoid it. Sadly it's becoming increasingly common, even on Steam, despite it not really having much effect on the availablilty of dodgy versions.