2a css meaning

1.parent {
2    background: red;
3}
4.child {
5    overflow: hidden;
6    height: 0;
7    background: blue;
8    -webkit-transition: all .8s ease;
9    -moz-transition: all .8s ease;
10    -ms-transition: all .8s ease;
11    -o-transition: all .8s ease;
12    transition: all .8s ease;
13    color: white;
14}
15.parent:hover > .child {
16    height: 30px;
17    display: block;
18}