The phrase describes the specific, punishing experience of performing calisthenics (pull-ups, leg raises, muscle-ups) on a single metal bar while the ambient temperature is dangerously high—typically above 90°F (32°C) with high humidity. The "hot" also refers to the bar itself becoming scorching to the touch.
In actual prison slang, a "hot one" refers to a murder charge, so be careful mixing those terms if you aren't intending to reference serious crime. for a TikTok or Reel using this theme? one bar prison hot