Burstable RAM is the amount of physical memory, available in the server in a shared environment on a temporary basis for any sudden load. Burstable memory us used when you are planning to add many shared servers in a dedicated server using isolation or virtualization software.
It is useful to have burstable memory, especially in a shared environment to handle any sudden increase in the load caused by shared users when they try to use more resources than allocated.
Burstable RAM is available to the server for a shorter period of time. You can not use them permanently and it is only available to handle small-time loads like software upgrades, database updates, and similar load cases.
For instance, in cPanels or similar web hosting control panels, we have a common 90 seconds availability of such burstable memory, if the user tries to use them for more than 90 seconds then they are temporarily suspended to protect the other shared users on the same server. If the same issue is happening with a client more often that means either they need to upgrade their server to a higher plan or something is wrong with their site.
This is Burstable RAM, also known as burstable memory, extra memory, and focused memory in the servers.