LESS
In plain terms
A CSS preprocessor similar to Sass with variables and mixins.
Going deeper
LESS adds variables, nesting, and reusable mixins to CSS, compiling to standard stylesheets. It was an early popular way to make CSS programmable.
Under the hood
Can run in-browser or be precompiled; similar goals to Sass with slightly different syntax and a JavaScript implementation.
Examples
Legacy Bootstrap 3, theming, maintainable stylesheets.
Think of it like...
Like a notebook of reusable style snippets you stamp into plain CSS.