what happens when you set a bootstrap class to bg cover code example

Example: background image for div in grid system bootstrap

.img {
    margin-right: -15px; // Remove right gap
    margin-left: -15px;  // Remove left gap
    padding-bottom: 62.5%; // ratio is 16:10
    background-position: 50% 50%;
    background-repeat: no-repeat;
    background-size: cover;
}