Please explain what it means to 'ensure' a master module.

I have just stumbled upon this project and have read to the 3rd paragraph of the project page at:

https://www.drupal.org/project/master > Master execute

It would be good if the text could have further info to explain what this sentence means:

drush master-execute ensures all master modules and their requirements by enabling them.

What does it mean to ensure a master module?

I assume it means 'ensure that the module and it's dependencies are present'. But, as a drush newbie, not so sure.