Seth Meyers is trying to understand teens. In the new “Late Night” segment “Seth Explains Teen Slang,” the host breaks down some popular teen sayings that might go right over the head of those not regularly keeping up with Kylie and Kendall Jenners’ Instagrams. (Which, by the way, you should be doing.) Let’s just say not all these definitions are exactly spot on. But if you don’t know what “on fleek” means, we really don’t understand how you’ve been surviving on the internet.