The war asset readiness rating is an indicator of how good your war assets will perform in the final battle. So the effective power of your war assets is the amount of war assets multiplied by the readiness percentage.
The Readiness Rating Drops Over Time
The readiness rating drops over time even when you are playing singleplayer. You can’t influence the readiness rating in the singleplayer, you can only gain war assets. The readiness rating is broken up into systems and you have to take care of all of them.
My readiness rating drops about 4% per system per day so if you are actively playing mass effect 3 you have to make sure to keep up with increasing your readiness rating.
Increasing Your Readiness Rating
When you play multiplayer your readiness rating goes up depending on whether you finished the challenge, what challenge it was (bronze, silver or gold) and where it was.
If you choose specific maps to play on your readiness rating will only go up for that system. If you go for random your overall readiness goes up. Random makes it harder to complete challenges because you won’t know what you are up against, but random location will give you a 10% experience increase and much better readiness increases. A 7% system increase is about equivalent to a 5% overall if you go with random. That means random is 500% better than specific locations.