A tubular magazine is just that, a tube under the barrel of a rifle that feeds the ammo. It's like the tube on a pump shotgun, only made to feed rifle or rimfire ammo. The GSG uses a detachable magazine so it has to be 10 rounds or less.
http://www.marlinfirearms.com/firearms/selfloading/60.asp
That is a tube fed 22
http://www.marlinfirearms.com/firearms/selfloading/60.asp
That is a tube fed 22