
As simple as snow… put your also known as CSR file in the text field below, hit button and review it's content (subjects). If everything is ok — put your key in a safe place and send CSR to authortives for signing.
Most common case for generating CSR is getting SSL certificate for your domain. If so — carefully review "Common Name", because it's most likely what you need. It must 100% match the hostname you need to secure with SSL. BTW, I would recommend using for ordering AlphaSSL or RapidSSL certificates.
Do not put here your private key! Actually, do not put it anywhere except place it should be ;—)