Means how much stuff you can still download for Cydia. This doesn't include themes, it is for Tweaks, System apps, and stuff like that. And that is a low number. For me my storage is about 89 Mb. And the higher it is the quicker everything should run.
You will notice it when you transition from app to app. meaning if you were to go into settings and go back out and so forth that transition would look natural not choppy...
It probably isnt much faster. The memory in the 2g limit it more than what's installed on /. There used to be a package on cydia called sysclean or syspartition that deleted a bunch of forgein language and misc stuff. I don't know if it's still on there or not cause I manually delete everything.