The title is not what it shows, that meetings are booked more on Monday 11am doesn't mean that people prefer or are more likely to show up. I guess they don't have data on attendance, only on booking.
I was confused by that terminology at first, too, but it appears their product has an "offer" phase where the meeting organizer suggests multiple times, and a "book" phase in which the invitees accept the meeting and it's booked. Which doesn't necessarily mean that the meeting is attended, but it is a higher level of confirmation/buy-in from the invitees than what happens at my work (FANG) where organizers just throw meetings on everyone's calendar.
