It's removing parts of the game for the sake of making it more accessible, easier to get into. Like merging some stats into a single one, removing certain skills or armor options.
I'm reluctant to call Tiberium Twilight a streamlining effort of Command and Conquer, but that was more of a butchering. Deus Ex Invisible War was streamlined in the sense that ammo types were replaced by a single, nanite ammo type, augmentations were simplefied and weapon modification almost removed. That's probably why so many gamers don't want streamlining, usually it's not a good idea.