Ordinarily I'd suggest jQuery because I'm jQuery-happy, but I think this can be achieved with pure CSS.
After some digging,
Hint.css might do the trick. It even comes at the luxury of being hosted
via CDN here, so all you have to do is link to it, then follow the instructions on their site for implementation:
Code:
<link rel="stylesheet" href="//cdn.jsdelivr.net/hint.css/1.3.0/hint.min.css">
A jQuery alternative such as
Tooltipster is more powerful, if you're interested in some beefier tooltips. It's also hosted
via CDN at this link, if you don't want to host the files yourself.
(If you're curious as to the benefits of using a CDN versus hosting files like this yourself, you can
give this a read, or Google around.)
(For further tooltip alternatives try searching up 'tooltip libaries' or 'tooltip plugins'. Real quick I also found this list toting to be the
Top 10 Best jQuery Tooltip Plugins.)
Let me know if you need some more specific help! If you want a more DIY method of tooltips, it's basically done via CSS with the
:hover psuedo-class. It wouldn't hard to write something up yourself if you'll only be using it a small handful of times.