It helps speed up your computer, making it faster to open programs and multi-task with many programs open at the same time. My laptop could only be upgraded to 2GB max so this was left over.
Well most RAM doesn't come with instructions but I would be willing to do the research for you. It's really easy, usually on a Macbook or Macbook pro its located under the battery compartment. You need a small screwdriver like the ones used to adjust eyeglasses. Once you have opened it up, its just held in by some tabs. Just push the tabs back and the old RAM pops up or if you have an empty spot then you just slide that new one in securely then just put the screws back on. Turn the computer back on and it will automatically find the new RAM. What kind of Macbook do you have?