Some software allows you to download effectively the entire program, and only authenticate with a key right at the end, rather than needing a key up front. This can often be done so the user gets limited access to the software on a trial basis, and then pays for the full version later. Given this, if you can trick the software into thinking it's authenticated, then you could pirate the software.

You used to be able to pirate the software using an app called a key generator, or Keygen, which was a mini program that could generate license keys for the software so that people could avoid paying.

In recent years companies have added additional layers of security to try and make it harder to pirate software. This can take the form of various Digital Rights Management tools (DRM), which could include having to authenticate the install by having an account with the company that has you as a registered purchaser. This makes it much more challenging to pirate the software vs simply having to provide a key that looks correct.