I assume you’re confused about what a beer flight is. Here’s the wikipedia article, but it’s just a set of small sample sizes of various different beers, usually available at a brewery/winery/distillery so that you can try the various options. The joke is that green is secretly an alcoholic, because he’s drinking early in the morning.

A flight of beer is usually five or six smaller beer glasses, usually filled with different beers, on a wooden plank.