Depending on the virtualization software used, the host can isolate certain resources per VPS. In general 'guaranteed RAM' would be the RAM that would always be available in contrast to 'burstable RAM' which may only be available if it's not in use by another VPS at that instant.

For example, if the host (the physical server) has 1 Gb of ram split among two VPS's you would only have the 512Mb of RAM that's allocated for your VPS. However, if needed and the other VPS is idled for a moment the other 512 Mb might be available for a very short time.