Can someone please explain to me what LESS is and how its used? I know it has something to do with CSS that's it though.

I use Sass myself, but it's the same basic idea.

What happens when you use a CSS preprocessor is that you break up your code into different sections (such as 'modules', 'layout', 'variables', etc) to write your code. Every time you save a file, LESS recompiles your code into a single stylesheet.

In terms of what it allows you to do, like Jesus said, it makes your CSS more maintainable and extendable.

As an example, say you have a site and a designer has chosen a particular colour palette. With regular CSS, you would have to copy and paste those colours (#13ea43, for example) everywhere that you want that colour to appear. Then, if a change is made later on and someone decides to use a different colour, you have to replace every single instance of that colour. With LESS, however, you can create a variable ($main-color: #13ea43, for example) and then just use that variable name wherever you want it to appear. Then if someone decides to change that colour later, you simply have to replace the variable value with the new value and it will take care of the rest for you. More complicated examples include entire element styles (box shadows, border radii, background colours, etc) that can be used in several locations and easily modified without copying and pasting everywhere. LESS also allows you to use functions and conditional statements (these are only available prior to compiling, however), which makes writing CSS quite a bit faster.

There's a lot more that preprocessors can do, but if you are serious about front-end design, it's definitely something to look into and learn. I don't think I can go back to regular CSS now! :)