An exhibition caseback is a watch's back cover made from a window of glass — almost always sapphire crystal — so you can see the movement inside. Instead of a solid metal back, you get a clear view of the gears, springs, and rotor doing their work.
Why watches have them
Manufacturers use them to show off the movement. Decoration like Geneva stripes, blued screws, perlage, and an engraved rotor only matters if someone can see it. An exhibition back turns the mechanism into part of the watch's appeal. You'll find them mostly on mechanical and automatic watches, where there's actually something moving to look at.
The trade-offs
A display back isn't free. Sapphire is harder to seal than solid steel, so exhibition-back watches often carry lower water resistance and are more exposed to pressure and impact. Many serious dive watches skip the window entirely and use a solid, screwed-down back for strength. You're trading a bit of ruggedness for the view.
Solid vs. display
Neither is better. A solid caseback can hold an engraving, seals against water more easily, and often sits more comfortably against your wrist. A display back gives you a reason to fall in love with the machinery. If you own an automatic, a display back is a great way to watch the rotor swing as you move.