The main system memory of the computer (the Ram) actually works quite slowly in relation to the speed at which the processor runs. This means the processor often gets ahead of the Ram and has to idle around waiting for next bit of data to arrive. Cache Ram uses memory chips that are several times faster than ordinary Ram and are better able to keep up with the processor. Unfortunately, it's too expensive to replace all the system memory with faster cache Ram so most computers have only 128Kb or 256Kb of cache Ram, which is hopefully enough to stop the processor being delayed too much while waiting for memory to catch up.
