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.

> I don’t understand what a ‘packer’ is

You've found the right series of articles then!

I read it - I did't find an explanation of what it was that he was building, just how he was building it.

Think of it of decompression software bundled along with a compressed version of the program you actually want to run. So execution starts in the uncompression part, it unzips all the code into memory, and then starts running the program you actually cared about.

See https://en.wikipedia.org/wiki/Executable_compression . They have been around since forever.

Packers were common in the 80's and 90's to save disk space. I mostly remember them from Amiga warez and demos.

It compresses executables.