Remember, the CPU is very fast, and RAM is slow. When the CPU needs something from RAM, it has to wait around. This means your CPU has nothing to do while it is waiting on RAM.

The cache, is faster than RAM. It is constantly paying attention to what the CPU needs, and tries to have it ready ahead of time for the CPU. You could compare cache to a waiter in a fine restaurant, who is always anticipating your next need. When you need something, the waiter is right there with it.