Training magazines, also known as dummy mags or blue mags (not as common), are made out of plastic (blue, of course), dont hold any ammo, and are weighted to simulate a loaded magazine. Theyre for use in guns being used for weapons retention or disarming training, or any other training that would require an empty weapon. I've seen them in brownells and dillon, various other places as well.
Of course, there are baseplates made in the same color, defeating the purpose, and other colors as well, of course. They're handy when you are looking for your magazines after a match, especially if your outdoors.