If you're looking at Linux based VPS', it means that you have a certain guaranteed amount of RAM (i.e. 64, 128, 256) and you have burstable amount of RAM (i.e. 64 MB to 1 GB). Guaranteed amount RAM means that you will always have that amount of RAM available on your VPS - no matter what. Burstable RAM means that if there is free RAM on the server and your VPS needs some additional RAM, it can burst the amount that is available on the server. In this case, if there is more than 1 GB of RAM available on the server (free), your VPS would only be able to burst up to 1 GB of RAM.
As for the Windows based VPS', it's not possible to burst RAM, so you'll only have guaranteed amount of RAM available.