Freeplay is the option you choose when you want to play the game without the campaign. It's where you can setup a match, change settings like FF on/off and such and just any map, without story tied to it. You may still see the cutscenes, but the map is not tied to a campaign. Also, the options available to players are much more open. In-game team swapping is also available in this mode.
In other words, this is the mode players will play, when they want to play a a more tradition MP style game.
Odd. I thought this was stopwatch mode.
I have played it the only way it should be played - single player.
That's the way it definately shouldn't be played, if you ask me.
At least the more recent CODs have been mostly focused on the multiplayer.