I've seen it around a lot, but cannot figure out what it's called. I assume it's a kind of compression fitting.
From a comment by Jimmy Fix-It:
This looks like a simple "squeeze" arrangement; where the stepped bevel at the back of the nut just squeezes the tubing slightly against the integral internal barb. No official name that I know of, but definitely a low pressure application. If anything, I would call it a "low rent compression connection"
It looks like Teflon tube; if it is you make the flare with a heat gun and a swage tool. Make sure to put the coupler on first or you will need to cut the flare off and start over. Also don't get the Teflon too hot, it just needs to be pliable enough to stretch and once cool it will hold its shape.