What is a Resale Certificate?

A wholesale supplier asked me for a copy of my resale certificate. My questions are:what do I have to do after I fill the resale certificate st-120 form? Do I have to summit it to the "New York State Department of Taxation and Finance" and wait fora response? Or just fill it and send it to the wholesale supplier? I think just have to fill it and send it to the supplier but I'm not sure.