The basic principle is that it adds small boxes with unique ids (numbers) behind every link, form field and frames on a webpage. You just have to type in the id and press enter (default mode, there is also an automatic mode to choose) to trigger the corresponding action i.e. following a link, pressing a button or selecting a text field, etc. The ids for frames are always place in the top left corner of the frame and they have a different background color.
Mouseless Browsing is an add-on for Firefox browser that lets you browse the web page using only keyboard. It looks little complicated at first sight, but soon we will realize how easy to use this for navigating web pages, selecting a text field, pressing a button, etc