what is the box-sizing in html code example

Example: textarea { -webkit-box-sizing: border-box; -moz-box-sizing: border-box; box-sizing: border-box; width: 100%; }

textarea {
    -webkit-box-sizing: border-box;
    -moz-box-sizing: border-box;
    box-sizing: border-box;

    width: 100%;
}