RAM is an abbreviation for Random Access Memory. This is a sort of computer memory, which, in contrast to other storage devices such as harddrives or DVDs, allows the information to be accessed directly without reading the previous content located inside it. Anytime an app is started, it is stored inside the RAM, as it could be accessed much quicker than if it was read from another media. With regards to the hosting service itself, additional RAM means that more web apps can work simultaneously on a given web server, especially when they're resource-demanding and are accessed by a significant number of people simultaneously. Unlike a shared web hosting plan in which the system resources of a given account could be flexible and often depend on what other users consume too, a VPS features a guaranteed amount of RAM that can be used at all times. That memory is allocated to one hosting server only and won't be used by other clients even if it's not in use.