“Levain” is a fancy French baking term (pronounced the same as your friend Herb Levin’s last name) that’s basically a fermentation kickstart for sourdough making. Someone asked so now you’re going to suffer through some goddamn manslplaining.
Like a lot of things in our weird new “sourdough is a trend” world, hipsters (and people who want to seem like hipters) make a big fancy fuss about levains, and all sorts of shit you should put in them and do. So, in an effort to bring some regular-guy to all this, let me dispel. 
It’s a HIGHLY technical process in which you make a small amount of very wet dough: some flour, a bit of your starter culture, and some water. Your bread recipe will guide you how much you’ll need; I usually make 150% of what’s needed, and give the rest to a friend- a new starter
Add some water, stir and adjust until it has the consistency of a nice tile grout. If you’re not into tile installation, make it like a nice cottage cheese. If you don’t know what that is, I mean, who doesn’t know what cottage cheese is like. Come on.
OK, okay. Maybe in some places there’s no cottage cheese. So you want it to look like this. Yes the undercounter lights make my hands look like I’m fucking 90 years old, or a victim of horrible nuclear accidents. Maybe I am.
Now, cover it with a plate, and leave it for a couple hours. Like everything with natural leavening, all sorts of factors including temperature, acidity, local water contaminants, and of course your microbial sample will change the time and technique needed. But that’s the point.
OK that’s it. Now you can toss “levain” into your skype groupchats and seem like a fancy-ass artisanal baker. If that doesn’t work, try “banneton.” I’m not going to explain that that’s just French for “basket” because it makes it seem less cool. /end
