Border radius controls how rounded the corners of a box look. It is like choosing whether a paper card has sharp corners or corners softened with a hole punch.
A small radius can make a button feel gentler, while a fully rounded shape can make a pill-shaped label. The value changes the corner curve without changing the words inside the box.
Use corner shapes consistently so controls feel like they belong to the same site. Very rounded corners are not always clearer; choose a shape that suits the size and purpose of the element.