Subprofiles makes it possible to provide different flavours, called subprofiles, of a given installation profile. Subprofiles are defined in the profile's .info file, and consist of a set of features available for enabling, with a default enable status. In an interactive install, the site configuration form includes elements to select which subprofile to install and can further specify exactly which of the subprofile's features to enable.
You won't typically need to download and install this module--if it's needed, it should be already packaged with the version of Drupal you're installing.
This module is used to create a blacklist of passwords that will be restricted from being used. So for instance if you don't want weak common passwords like "password", "123", "god", "love", etc.