Corner feel is the impression created by the way a design's corners are shaped. Sharp corners can feel crisp, while rounded corners can feel softer, like the difference between a square envelope and a smooth river stone.
On a site, buttons, cards, and pictures may all use a similar corner style. Consistency helps the parts feel like they belong together.
There is no single correct amount of rounding. Choose a shape that suits the product, then use it in a steady way. Corner style should not make a button's purpose or boundary hard to see.