Something for all the Star Trek fans out there.
For those not Star Trek fans, an explanation:
After sitting through four years of dull interspecies protocol seminars and tactical maneuvers groundwork, you successfully graduated from Starfleet Academy. As a newly commissioned Federation security officer, your first assignment is on board a Constitution-class heavy cruiser. You straighten out the collar on your crisp red shirt as you walk down the corridor toward the transporter. You’ve been summoned to provide defensive support for a landing party consisting of the top officers on board. This is your chance to shine.
Shine like a phaser set to “frag.”
Perhaps you should have gone into science.
In the Star Trek universe, the Red Shirt is the phase-cannon fodder obliterated on screen to alert the audience to the danger of the situation. It’s Roddenberry proclaiming, “We could have just killed one of the characters you cared about!” The Red Shirt is a sci-fi idiom for the anonymous, the expendable, the smoking boots behind a boulder.
Sir Terry Pratchett dedicated Guards!, Guards! to, among other groups, Starfleet security ensigns. Enjoy.
ADDENDA: David Bath has offered the following infographic.