mbps = mega bits per second. That is the information flow rate (bandwidth). Higher rate means better resolution. More commonly, when you play MP3 files, you see it's compressed to 128, 192 kbps, etc. For DVD, I am not sure what the standard rate is, but 3 mbps is probably low, and 8 is high.
edit: one more thing. Usually they are encoded as variable rate unlike MP3 files. That means high action scene will require higher rate. For drama, probably, 3 mbps is OK
edit: one more thing. Usually they are encoded as variable rate unlike MP3 files. That means high action scene will require higher rate. For drama, probably, 3 mbps is OK