The SSL Key, also known as a Private Key, is the secret key associated with your SSL certificate and should reside securely on your web server. When you create a CSR your web server will also create a SSL Key. When your SSL certificate has been issued, you will need to install the SSL certificate onto your web server - which effectively marries the SSL certificate to the SSL key. As the SSL key is only ever used by the web server it is a means of proving that the web server can legitimately use the SSL certificate.
If you do not have, or lose either the SSL Key or the SSL certificate then you will no longer be able to use SSL on your webserver.