Tooltips are a good thing for a better usability in webdesign – you find a good description about them in wikipedia: http://en.wikipedia.org/wiki/Tooltip. They are shown when you hover a link or menu element. With css you can modify the tip as you like.
and css only
A better way for all users would be to use css http://lixlpixel.org/css_tooltip/ and as whole popups here: http://meyerweb.com/eric/css/edge/popups/demo.html
Some love it some hate it
In case you design a menu an extra tooltip could disturb the user. Often tooltips are used to explain a link – but in a menu it’s clear and not good to use. What can you do – where does they come from? Each link with a title attribute will show a tooltip. So the easiest thing to do would be to delete the title attribut within the link.
Special Joomla: In the basic mainmenu there are no titles – fine – but some people think they need them, so you use the extended menu module to add them.
For SEO reasons, titles can be useful. They are good if they give extra information about the link – but in a menu you should avoid just to repeat the link name in the title.