Basically it is a CSS compiler, that enables you to use variables, functions (mixins) and nesting inside of your CSS. All you have to do is make a file, style.less, edit it using the less syntax (essentially the same as regular css, only better), then run
And it will generate a nicely formatted CSS file.
If you want it to update the style.css as you edit the style.less file, run
lessc style.less --watch
You need to have Ruby installed to run this program.
Here is an example of a .less file I am working with: