Audience retention is the percentage of a video viewers keep watching at each moment.
Studio shows retention as a graph across the video's length. A steep drop in the first 30 seconds usually means the opening didn't match the title or thumbnail. Flat sections mean people are engaged; bumps mean viewers rewatched a part.
Retention is one of the strongest signals YouTube uses to decide whether to recommend a video. Compare videos of similar length on your own channel rather than chasing a universal 'good' number.