First, it's DLC not DCL,, second, it stands for DownLoaded Content, and third, It is stuff that Turn 10 made after the first release of the game. In each pack, they choose one car to be free, in this case the 07 Seat Leon, and the rest for X amount of Microsoft Points, depending on the amount ofcars in the pack. They also put up several free packs, like the Nissan Tournament Pack, to promote new tournaments etc. If you download the single free car, it enables you to see all of the other DLC to that point in time (so the Seat Leon enables you to see any car in, and prior to, the September Car Pack). Turn 10 have also released a DLC Track, Twin Ring Motegi, in four ribbons (layouts). Hope this clears everything up for you.
PS. I'd recommend downloading any free content if you don't Need the cars in the pack, however, I would download the track, as it is impossible to compete or spectate, a race that is on a DLC track that you haven't downloaded, and more and more people are learning and racing on the track.
2 and a half years of Forza and I'm still one of the little guys :D