Someone else may be able to describe it better than me, but the BIN is the tune. It has all of the parameters required to ensure that the engine is getting the proper amount of fuel and ignition timing based on the data it's getting from the various inputs. If you are going to start tuning, it is best to either know what tune (BIN) is already in the ECM or just start fresh with something else. You only start tuning once you've done a datalog with whatever BIN you're going to use as a starting point. For the 7747 ECM, there are also a few changes you should make before starting to datalog to get the best results. There are a few very good tutorials here and lots of good info from some experienced tuners to go through that will help get you started.

Since you have an Autoprom, you can read the chip and get the BIN off of it. If the chip has a label on it, you might be able to find a copy of it here as well. Depending on the specifics of your setup, you may find that you would want to start with another BIN that's closer to your needs, or there might be someone here that has a good starter BIN for your combo. I know that people have put TBI on the 258 before, so someone might have something that's pretty close to what you need.