For those unfamiliar, Less & Sass are CSS transpilers that allow you to write CSS in a more elegant manner. This lets you do things that aren’t normally supported, like nesting CSS rules. Once finished, these CSS transpilers “compile” your code and convert it to normal CSS.
There are 2 major CSS transpilers right now: Less and Sass. Sass is the more popular one, but learning Less first to be easier. It doesn’t matter whether you learn Less or Sass first. They’re extremely similar, so once you know one, you pretty much know the other.