FOSS gamedev and creative worlds

What do they do and how do you use them? I've tried using them, but it just doesn't work.
I'm thinking it's a exchangeable crafting thingamabob, but I'm most likely wrong.
HALP PLS I FAN

Groups are sometimes used for crafting receipes, but that's not all they're good for. They also define how easy they are to dig (and which tools), if they're falling nodes, other ways of interacting with the game etc. Just take a look at doc/lua_api.txt It's pretty extensive.

Here's a short explanation what groups are:
http://rubenwardy.com/minetest_modding_ ... tml#groups