Premium content gives you more content for the game (you usually purchase it). It's different for all games, but what's usually in there is more maps, weapons, characters, extended story, stuff that generally improves the game for the player.
And just to make it clear, you dont need to pay to armor games anything, armor games is complelty free. The developeres of the games sometimes offer it as way to support them and there games making.
AG itself as a website is free to play, there is no subscription or price to have an account on here.
But; on certain games, particularly MMO's there is premium content, which can be purchased to enhance your gaming experience on said game.
In some cases the premium content is the only thing that makes the game playable. I can think of at least one MMO know as EdgeWorld that went this way. v_v
