How can I add a text which appears when the mouse in on a category of the menu?

Hi korbakis. You might try the Simple Tooltips plugin. Pretty easy to implement as shown in the documentation.

In Settings – Simple Tooltips, the class is .nav for the main header menu.
In your menu items, the Title Attribute is what is displayed in the tootltip, and the CSS Class is “tooltips” (without the quotes).

Glad I could help.