Burst RAM

Burst RAM is the maximum amount of RAM you can receive when the physical resources are available, it is different then the swap memory
It will be 256 MB of ram which is granted for you, you will not get less then that, and 768 will be the Maximum limit if available on server (it should be available coz knownhost have 8 GB of ram on each of there server)
Burstable means that it is available for short durations of time when load spikes occur.