BurstRAM is a standard part of all VPS's powered by OpenVZ virtualization. It allows a VPS to use beyond its regular (dedicated) allocation of RAM.

Let's say your VPS had 256MB of dedicated RAM. This is entirely for your VPS, no one else on the node can use it up, it's like a private piece of RAM. Now, if your VPS has 512MB burst RAM, this means that while you have only 256MB guaranteed, you may use up to 512MB if there is unused RAM on the node. Of course, customers constantly using their burst RAM will be contacted, because as the name suggest, it's designed for 'bursts' of high activity on the VPS.

The above information applies to all VPS hosts, a little note just for Fanatical: we always monitor our nodes and never full them beyond 50% RAM usage during peak, thus, burst RAM is always available when needed.