CSS stylesheets can become hard to manage on large projects. LESS is a solution that developers use to make their CSS much more efficient. As many designers know, stylesheets can become complex on big sites. It can become a headache to manage hundreds or thousands of styles. LESS allows you to use plain CSS, but also add programming features such as variables, mixins, and functions.
LESS is an extension to CSS, so it works perfectly with normal CSS, and the extra features it adds use existing CSS syntax. This makes learning LESS a very popular choice for coders, including those who build popular design frameworks, themes and templates.