Heh - these guys are quite simple compared to some others.
The red screw is the attachment point for the skimmer to the base plate. Unscrewing this screw lets you take the whole assembly (except the base plate where the pump is mounted) out of the sump so it can be cleaned periodically for maintenance. Those red screws can be a bit fragile after being in salt water for a while, so take care when unscrewing them.
The plastic cylinder is a muffler for the air drawn into the skimmer. If you disconnected the plastic cylinder and it was just a plastic hose, you would have a horrible sucking sound as the skimmer draws in air to make bubbles and produce skimmate. When the skimmer is running, place your finger over the hole in the top of the plastic cylinder and watch the entire skimmer be impacted.
Its a nice skimmer! Good Luck!