oh, you're not being stupid at all -- the funny thing is there is not much information about out there that it can be quite frustrating. a regulator is a device that usually has a gauge in a knob on it that allows you to control the pressure that is coming through. Many times all be combined with a moisture trap as well. However, some regulators are simply a screw in line with the hose to control the amount of air that's coming through. It's an important piece of equipment if you want to gain more control over your brush, but not absolutely necessary for general spraying.
some compressorswill will already have these built into them. What type of compressor do you have? let me define what I mean by "good source of air". big problems you can have is if you're a resource varies its pressure when you start spraying. I had a compressor like this, and it was very problematic. You press down on the trigger and get a big rush of air, and then the pressure would dropped to the pressure I expected to be spraying. Some compressors are lubricated with oil, and this can get into the airstream which then will mix with your paints. This can be problematic as well, and this is why a moisture trap of some sort in line can be really necessary. Additionally, depending on the compressor, its motor, etc. -- air can become heated and then as it cools will cause condensation on the inside of the tank. This is where a lot of the water can come from. Many compressors will have a release valve for water is trapped inside the tank and it's good to drain it every so often. These are just some of the things I can think about off the top of my head.