To give a little more in depth definition lets assume you were given 320GB of bandwidth in a month. This would be roughly 1mbit. But if your site was limited to only sending 1mbit at a time, it would appear slow if several cable modem/dsl users got on it at the same time. Instead you get 320GB of bandwidth per month, but you might be on a 100mbit port. This means if your site gets hit with a bunch of traffic at once, it can send up to 100mbit's during that time. Most of the time your site will be idle or low usage, so you can still stay under your 320gb allotment, but have the speed if you need it.
Burstable ram is much the same, but applies to a VPS. You may be given 128mb of ram. That means your VPS can use 128mb of ram no matter what. The server it's on might have 4GB of ram though. if no other users are using up the rest of the ram, you could technically use up to 4GB for short periods of time.