LESS is a CSS extension language that adds features like variables, nested rules, and functions to regular CSS. It allows defining reusable values like colors and spacing, writing nested selectors more naturally, and adding parameters to mixins with default values. LESS can be compiled to CSS or interpreted dynamically with JavaScript. Bootstrap is a toolkit from Twitter that provides base CSS and HTML for common interface elements like forms, buttons, tables, grids, and navigation to kickstart development and ensure consistency. It can be included by directly linking stylesheet files or using LESS variables and mixins. The document discusses how LESS adds features useful for managing CSS, and how Bootstrap leverages LESS to provide reusable interface elements.