The phrase "left fork" refers to the direction that you take when a path or road splits into two, and you choose the left side to continue your journey. Full definition
Every road taken leaves a fork unexplored, and the road taken to date seems incomplete, littered with unanswered and important questions. (educationnext.org)
I wanted to ask him if he was the first person who was ever be late for something, mostly because he is 20 years old, and in the same situation at his age most people are sleeping in beds without box springs, destroying microwaves by accidentally leaving forks in them while heating water for ramen, and discovering that things in your house stop working when you don't send strangers in businesses money in the form of checks. (sbnation.com)
After you pass the fourth cornfield, take the left fork — you'll know the one when you see it; you really can't miss it. (relevantmagazine.com)