If you mean the bulge near the top of a pint glass, it’s usually called a nucleation ridge or bead—and it isn’t primarily decorative.
The ridge can help agitate the beer as it’s poured or sipped, creating tiny nucleation sites where dissolved carbon dioxide forms bubbles. That can help maintain a lively head and release more aroma.
There’s a second reason for some classic pint designs: the bulged “nonic” shape makes the glass easier to grip, less likely to chip when stacked, and more resistant to being knocked over.
So the surprise is that the bump can be both functional glass engineering and beer engineering—depending on the design. 🍺