Engagement calculates the number of bookmarks/notes/comments a session has compared to the amount of times it was scheduled. Sessions with high numbers of bookmarks/notes/comments compared to the number of users that scheduled will have a high engagement level.
The most popular sessions may be fun and interesting, but some smaller sessions may have a much more profound impact on attendees. The Engagement report highlights sessions that stand out, either due to the topic or the speaker presenting it.