For the basic tooltip, you want:
<div title="This is my tooltip">
like:
.visible {
height: 3em;
width: 10em;
background: yellow;
}
<div title="This is my tooltip" class="visible"></div>
For a fancier javascript version, you can look into:
https://jqueryhouse.com/best-jquery-tooltip-plugins/
The above link gives you 25 options for tooltips.