[The_Amp_Walrus]

Context free means that the language/program can be described using a "context free grammar": https://en.wikipedia.org/wiki/Context-free_grammar.

Context free grammars are more expressive than regular expressions, but less expressive than full-blown programming languages.

If you've got a spare 100 or so hours and you're interested in learning more, there's a good lecture series on YouTube about this stuff: https://www.youtube.com/watch?v=TOsMcgIK95k&list=PLbtzT1TYeo...