The code to achieve this would be something like this:
a {
position: relative
}
a span {
display: none;
position: absolute;
bottom: 0;
width: 100%;
padding: 10px;
background: rgba(255,255,255,.8);
}
a:hover span {
display: block
}
<a>
<img src="http://placehold.it/300x200">
<span>caption</span>
</a>