[rustybolt]

From what I understand, it's something that 'packs' an application, either to preserve space or to obfuscate it (making it a little bit harder to reverse engineer it since you have to peek at the extracted contents in the memory instead of at the file contents).

So when you would run a packed application, there application would essentially start to unpack an encrypted/compressed blob of memory, and then jump to it once it's unpacked.