amazon ec2 - Setting Up ELB with SSL - What is Backend Authentication?

I started setting up Amazon's Elastic Load Balancing
Service for my server pool and I need to setup HTTPS/SSL. I have all my SSL Certificates
setup, but then I come to the step for backend authentication and I'm unsure what
certificate is required with the "Backend Authentication".

Is it my sites private key, public key, or do I
need to generate a new key on the
server?

Thank you for the
assistance.