In Mass Effect 3 for the PC, PS3, and Xbox 360, one of the primary ways to see the different endings to the game is by gaining a certain number of effective military strength.
One way to increase your military strength is by increasing your galactic readiness rating. The galactic readiness rating is only raised by playing the Galaxy at War multiplayer games (which require the Online Pass), or by playing a couple of the games for iPhone and iPad. These apps are called Infiltrator, and Data Pad.
Raising your galactic readiness rating simply lets you apply more of your military strength into effective military strength. Playing no multiplayer games leaves your readiness rating at 50%, meaning that a 5000 military strength score is reduced to 2500. If you play a lot of multiplayer games and increase your readiness rating to 100%, then you would get your full 5000 points of military strength.
Note that you shouldn’t need to worry about this rating if you don’t want to, since you can increase your military strength by collecting war assets in single player, however you’re going to need around 8000 military strength, since playing no online games will take your effective rating down 50%.