It means (most likely) that you cannot surpass 3gb of bandwidth per month. So every time data is transferred to your computer you are hacking away at that 3gb. Most webpages are only a few hundred KBs, but if you watch a LOT of video you can definitely hit that limit rather quickly. That also depends on the quality of the video. Generally YouTube videos are around 5+mb (depending on length), but if you were to watch a streaming high quality show everyday for an hour I'd bet you'd hit that limit quickly.
I'm dial-up so at least I don't have to worry about that...LOL
Leo Laporte has been talking about the bandwidth cap issue a lot lately on his radio show The Tech Guy (TechGuyLabs.com). Listen to some of the episodes from the past few weeks for discussion, it's really interesting. Comcast is considering something like a 30gb monthly cap which could really disturb those who consume bandwidth by watching videos, and downloading large files.
3gb sounds extremely low though. I could hit that using dial-up (maybe

). Unless it's 3gb a day...That's not bad.