Rounded corners is what we call the corners when they appearwell..rounded!
This functionality, if I may call it that, is accessed via a css property border-radius that allows us to give any element rounded corners, thus allowing us to thoughroughly shape the desired element.

Below is a div element styled with a background colour and border of different colour.

#divroundedcorners {
    background-color: lightgreen;
    border-radius: 7px;
    padding: 5px;
    min-height: 100px;
    margin: 0 3em;
}

In case we only want to cut one corner, we simply need to set the remaining corners to 0, like so:

#divroundonecorner {
    background-color: lightskyblue;
    border-radius: 0 27px 0 0;
    padding: 5px;
    min-height: 100px;
    margin: 0 3em;
}