Confused by create module tutorial statement: "function onthisdate_admin" is not a hook

In the create a module tutorial, on the page "Create a module configuration (settings) page", under "Create the configuration function" (at http://drupal.org/node/206761 ) it says:

We'd like to configure how many links display in the block, so we'll create a form for the administrator to set the number of links. This is done in our function onthisdate_admin. Note that _admin is not a hook and we could have used whatever we wanted there.

If the onthisdate_admin php function is not a hook and can have any possible name, then who is calling it, how do they know what the name is, and when does it get called?

Thanks in advance
Ben Slade