Woow!!! That is the word that best defines Less.
Now, Alexis Sellier (alias cloudhead) has created the amazing Less tool which can be defined as "The dynamic stylesheet language".
LESS extends CSS with dynamic behavior such as variables, mixins, operations and functions
Imagine a web project with many lines of CSS defining your nice styles about buttons, borders, backgrounds, etc. Image you have a nice #aaaaaa color shared among many elements (borders, text color, ...). What happends if you decide to change that color? you need to rewrite the color code on every place you put it.
The magic of Less allows you to define a color variable and use it around. Later if you decide to change the color, simply change the variable value.
Instead create CSS files you need to create LESS files, they are similar to CSS bit more "powerful" (you can use variables, functions, ...).
Less library is responsible to read the LESS file and create a CSS file for you.
In a production environment, once set the LESS file, it is better to compile it to a CSS and incluthe this last.