Similar to cold calling. The person you're calling still doesn't know you or expect your call, but you happen to know someone who they know and like. By referral the ice is broken, and the call is warmer in temperature.
Salesperson: "I need to make a few calls, was there anyone at that networking breakfast who you think I should try?"
Friend: "Ya, try Pete Mullholland, he's the manager at Vandelay Industrial Supplies--we got along great"
Salesperson: "Finally I'm warm calling for a change"