The term "cold market" refers to a group of people who have not shown any interest in your product/service previously. You have no idea if these people are interested in your product/service already, as they haven't opted-in on a lead capture page or have reached out to you showing interest. Often, this group of people will be totally new to you or your product/service.
When interacting with your cold market, you'll want to use different language to engage them with your product/service than you would with your warm market.