What is a group head and why does it matter?

The group head is the component where the portafilter locks in and through which water flows into the coffee puck. It's thermally massive in quality machines to maintain consistent brewing temperature. The E61 group head (found on Rocket, ECM, and Lelit machines) is the most common prosumer design — it's thermally stable, durable, and easy to service. The quality and thermal mass of the group head directly affects temperature consistency from shot to shot.