“What is it? What does it do?”
Less is a CSS pre-processor. It is fairly easy to learn and extends the CSS language adding features that allow variables, mixins, functions, and many other techniques allowing the user to make CSS more maintainable, themable, and extendable.
Less can be downloaded and used freely, in whole or part, for personal or commercial use. Less can be used on the command line, downloaded as a script file for the browser, or used in a wide variety of third party tools.