RAM memory stands for "random access memory" and is vital to a computer's performance. Think of it like a computer's "short term memory." The computer stores things there temporarily while it needs to think about them, and then swaps that information out with other data as it needs it. Just like with short term memory, when the power is turned off the things stored there go away. That's why people can lose their work if there is a power outage and they haven't saved their documents. They call this type of memory "volatile."
It is important for you computer system to have enough of this type of memory and to have it of the right speed. Basically, if the RAM is not fast enough it won't be able to keep up with the CPU (computer brain) that is requesting information from it. The result is that your computer would stop working. If you don't have enough RAM in your computer it will have to keep "swapping" with the hard drive (the computer's "long term memory") and the whole system bogs down.