In the single player game you control Shephard as they travel the galaxy collecting what is referred to as 'War Assets'. A war asset is something that will make a significant difference in the fight against the Reapers and can be anything from a special character to a full sized fleet of ships, each asset is given it's own point value indicating how big a contribution they will make. A progress bar is used to indicate how close you are to having enough assets to begin the fight against the Reapers. It sounds straight forward but unfortunately each asset is affected by what is known as 'Galactic Readiness', this extra factor determines how much of each assets value will be applied and is determined by how much you play the multiplayer game. As an added layer of complexity the 'Galactic Readiness' decreases over time.
So in order to do well in the single player you're forced to play online, don't worry though as Bioware have really made sure it's an enjoyable experience. Selecting a character class from the wide range of options you are dropped into a horde mode style of play and forced to face wave after wave of enemies, occasionally completing a mission with your allies where possible. As you complete each round your character will gain experience and be given the option to purchase new items, like guns or armor mods. Given enough playtime you will even be able to promote your character so that they become a war asset.
If you're still concerned about the readiness decreasing when you aren't playing online then you'd probably want to investigate the 'Mass Effect 3 Data pad'. This handy little app has a number of features including a fully voiced codex, messages that are sent from "characters" in game, and best of all a 'Galaxy at War' mini game. This mini game allows you to dispatch fleets of ships to key areas, liberating them from Reaper control and increasing readiness as they go. In some way it reminds me a of a browser based game called 'Planetarion'.
Single player, multiplayer and even external mini games all working together to immerse me in the conflict between the Reapers. It was an enjoyable experience and while I was lucky to have the time to play online, and an iOS device to run the app it's one that really elevated Mass Effect 3 in my eyes.
