When we use the border-radius?

Question

This is the quite hard value.

Answer 1

Your Question clearly indicates ,U haven't read the "Rounded Corners" lesson carefully.
Read the 👇lesson carefully to understand border-radius property:
https://www.sololearn.com/learn/CSS/2238/

Answer 2

Border radius property is used to make curved border.

This value can be given in pixels (px), percentage (%), points (pt), etc.

Syntax :
border-radius: 10px;

You can also make border circular using this property.
border-radius: 50%;

There are also variation, if you want a particular corner to be curved/rounded, then use
border-top-left-radius: 10px;
border-top-right-radius: 5px;
border-bottom-right-radius: 10px;
border-bottom-left-radius: 5%;

Answer 3

We use border radius if we want to give a round corner to a element.

border-radius: 100px; all corners
border-radius: 100px 200px; top-down and left-right
border-radius: 10px 20px 30px 40px; top right down left