Burstable RAM is the memory which is available beyond the guaranteed RAM.Burstable RAM is basically shared RAM amongst each VPS on the hardware node. It is divided equally among each VPS and is there for when each VPS needs it. For example your VPS might have 256mb guaranteed ram, and 1024mb burstable RAM.
This means that after you have used up your guaranteed RAM, there is still 768mb burstable ram available for burst usage – IF there’s enough free memory on the host server.Burstable RAM, means that you can go up to that much usage, but it must only be for a short time. In other words, you can burst to that plateau of RAM, but cannot always remain at that level.