What is LESS?
At a high level
The LESS website describes LESS as:
The dynamic stylesheet language
LESS extends CSS with dynamic behavior such as variables, mixins, operations and functions. LESS runs on both the client-side (IE 6+, Webkit, Firefox) and server-side, with Node.js.
Thankfully the Web Workbench includes all the requirements and is a nice simple one install way of working with LESS.
Learning the syntax
The Web Workbench from Mindscape will help you code correct LESS code by providing intellisense and syntax highlighting but you will still need to learn the syntax. You can read the guide on LESS syntax from the source here: LESS Documentation.