Something that is "riveting" is so interesting that you can't look away from it.
An actor's performance can be "riveting". A novel or a real person in a documentary film can also be riveting.
This phrase comes from the word "rivet", which is something like a nail which holds pieces of metal together. When you see something "riveting", it keeps you from looking away or moving at all, as if your body has been riveted to your seat.