What is it?
LESS is a CSS preprocessor framwork with features such as mixins, variables and functions.
What is it like?
Nice and easy to use with a good clean syntax. I used LESS with several projects because it had good support within the Meteor community but for new projects I would opt to use the slightly more powerful SASS instead.
SASS/SCSS has overtaken this as the CSS pre-processor of choice. They are easier and more enjoyable to work with.