Simple analogy to understand this, it's easier if you take on real events, in daily life. For example in the work environment. Where there is a desk and a filing cabinet. Filing cabinets can be assumed as a hard drive, available files and information needed. We began to work, the files information will be excluded from the closet to the desk. This is to facilitate and accelerate access to required information. Thus, the desk can be analogous to memory on the PC.
With a simple analogy is, of course you can already imagine the difference in speed of access, between memory and hard drive. Also is looming, the importance of the availability of the amount of RAM, which is sufficient to serve the data and information required by the CPU.