It's a volatile memory. It temporarily stores the data in the RAM until it is needed. It's also a primary memory in the Computer.
It disappears when the computer is turned off: if you're working on a document you'll need to save it to avoid losing it. When you save a file, the data is written to the hard drive, which acts as long-term storage.
RAM is measured in megabytes (MB) or gigabytes (GB). Many people add extra RAM to their computers to improve performance.