Acording to Wikipidia, "Random-access memory (RAM /ræm/) is a form of computer data storage. A random-access memory device allows data items to be accessed (read or written) in almost the same amount of time irrespective of the physical location of data inside the memory. In contrast, with other direct-access data storage media such as hard disks, CD-RWs, DVD-RWs and the older drum memory, the time required to read and write data items varies significantly depending on their physical locations on the recording medium, due to mechanical limitations such as media rotation speeds and arm movement."
That explination will sound strange so think of it this way: your computer can only "check out" a certin amount of files from the harddrive to read at a time. The RAM is that person that "checks" the files out and reads them