Last December, a team of scientists made history by creating a fusion reaction that—for the first time ever—gave off more energy that it took to start. It’s a groundbreaking milestone.

We talked to two researchers who were part of that team—Sabrina Nagel and Matthias Hohenberger—about what exactly happened, why it’s been decades in the making, and why it’s such a big deal. This is everything you need to know about their team’s fusion breakthrough.