CA is Certificate Authority. Basicly, by trusting the CA (by installing the CA certificate), you trust any certificates it issues. You don't have to do this, but you'll get a warning saying that the root/CA isn't trusted.
More (technical/correct) info here:
http://en.wikipedia.org/wiki/Certificate_authorityEdit: You might also want to look up PKI to see how everything goes together. SSL keys for server validation are jsut one small piece of it.
