Ryan Reynolds announced this week that doctors discovered a polyp during a colonoscopy he had after losing a bet.

The actor and his fellow actor, Rob McElhenney, explained in a video that Reynolds bet McElhenney the latter wouldn’t learn to speak Welsh. After Reynolds lost the bet, the pair chronicled both of their colonoscopies, filming themselves before and after their procedures.

After Reynolds’ colonoscopy, his doctor, Jonathan LaPook, shared that he found a polyp, a discovery that was “potentially lifesaving.”

“I’m not kidding. I’m not being overly dramatic,” the doctor said. “This is exactly why you do this, okay? You had no symptoms.”

McElhenney also had three polyps that were discovered and removed.

But what is a colonoscopy? Why are they important? Here’s what you need to know.