

In my city people often stand with bikes or strollers in the door area. Out sometimes it’s just really packed. When the opening side is announced, those that stand in the way can already make way (if they are considerate to their fellows) or know that they can relax and don’t need to move.
The German city name Frankfurt isn’t unique. That’s why the cities often have an identifier. The biggest and most well known Frankfurt is located at the river Main and therefore called Frankfurt (Main). Main is also used as description in programming which is where the rest of the joke is coming from.