To expand a bit:
There's only so much sugar that can be extracted from grain, and that number is consistent from batch to batch for a given type of grain. Your efficiency is how much you manage to extract. You can figure out your efficiency by calculating the highest possible original gravity for a given grain bill and comparing that to the OG you obtain. Most software will do this for you. Your efficiency can vary greatly based on technique, but as you become consistent in your brewing habits, you'll find that your efficiency becomes consistent as well. From there, when you're figuring out recipes, instead of just taking a recipe and accepting the OG that comes from it, you can use your efficiency to alter the original grain bill. Not very efficient? Just scale up the grain bill and you're all set. Extremely efficient? Save money on grain by not buying as much. Again, software will take care of all of these calculations for you. It's possible to do them by hand, but I don't think anything is really gained by doing it that way - unless you like to have a complete handle on every step of the process.
Hope that helps!
