The dynamic stylesheet language.LESS is a dynamic stylesheet. It may be helpful in understanding LESS by thinking of LESS as similar to PHP. In much the same way that PHP is interpreted by the server and then outputs the corresponding evaluated HTML, LESS is interpreted by the server and outputs the corresponding CSS. It's that simple.
To be more specific, LESS extends CSS with a variety of additional features.
LESS extends CSS with dynamic behavior such as variables, mixins, operations and functions.
LESS runs on both the server-side (with Node.js and Rhino) or client-side (modern browsers only).
http://lesscss.org/