What does the term 'Bootstrap' mean? Referring to https://www.drupal.org/project/health_check

I am confused as to what 'If Drupal fails to bootstrap' means.

What is term 'bootstrap' referring to? Does it make a database call? From what I have read bootstrap can mean from the Drupal core.

So if the healthcheck module fails is it because it cannot see the database, will the module render a 200 or some other response code?
In computing, "bootstrapping" refers to software getting to a state to where it can perform the task being requested.

In the context of Drupal, it means the code starts and does everything up to the point where it generates the page.  It more-or-less includes


connecting to the database
reading configuration
loading modules


The bootstrap process is pretty much the same for everything request; what happens when a page gets rendered and sent to the browser differs page-to-page.

So, when "Drupal fails to bootstrap", it means that it wasn't able to get to a point to figure out what to do, and is considered a Fatal Error.  In my experience nine times out of ten when it happens in a production environment, it is because there was a problem connecting to the database.