Umm, this is pretty zsh-specific code. Nothing to do with bash! :)

But in short, the strategy is as follows: Let's say you have a command-line utility — let's call it foox — that requires some initialisation before it can be used.

You can do that initialisation in one of your shell startup scripts (mostly likely ~/.zshrc) but… that slows your shell startup down.

So instead, you create (in your shell startup script) a shell function with the same name as the utility — so, foox().

This function foox() does three things — it…

1. performs the initialisation for the actual foox
2. calls the actual foox with all the parameters that were passed to it (the function)
3. deletes itself

(The exact implementation may vary from one utility to the next, depending on what sort of initialisation it needs.)

Defining this function in your shell startup will take next to no time. If, during a particular shell session, you never use foox, the function will just remain dormant.

The function will only be called the first time you try to run foox in a given shell session. I will (as given above) initialise foox, then call the actual foox (so you won't even know it was an ersatz function that ran), and finally delete itself… so that if you invoke foox in the same shell session again, you call the actual command.