First crack, and second crack refer audible cues in coffee roasting. It can sound like popcorn popping. Sweet Maria’s in Oakland, California has a great video about what the coffee looks like at first and second crack here :
I have just posted the third in a series of videos from my interviews with roasters in Sacramento. In this one, master roaster Eton Tsuno and roaster Ryan Ausbun talk about their method and technique in coffee roasting. Eton does a great job of explaining how roasting affects the flavor, and Ryan backs up the explanation while he roasts espresso.