POSE, or the "Palm OS Emulator", is a bit like MAME or Stella or other game machine emulators. It's a program that simulates a Palm machine down to the hardware level, allowing for detailed testing and debugging. It allows you to have an entire virtual Palm on your desktop machine. This is great for testing both your own programs and those of others.
More recent Palms are not supported by the Emulator, so Palm came up with a "Simulator" that emulates at least the software of a modern Palm. Sadly, they have not released a Linux version or the source code thereof, but they are apparently working on an Emulator that will be portable.