what is media only screen in css code example

Example 1: css media screen

@media (max-width: 991px){
    .mobile{
        display: block;
    }
}

Example 2: css @media

/* When the width is between 600px and 900px OR above 1100px */
@media screen and (min-width: 600px) and (max-width: 900px), (min-width: 1100px) {
  div.example {...}
}