When you enable a new Site User to register their own account on your site and they click to "Create Account" the user is taken to the registration page and required to fill out a number of details before the account is created. This page may also show an option to choose a "Boss". A "Boss" can be used within some advanced approval workflows.
This option on the registration page enables them to choose from any of the availble Site Users that are included within the Site Group selected as options for a boss for Self-Signup users.
If you would prefer for the customer to not have to select a boss, this option can be disabled by going to SiteName > Settings > General and in the section labeled "Basic Settings" you will see a field called "Boss for Self-Signup Users". Deselect any options that are chosen in that field and save the settings. When you revisit the Self-Signup page, the option to choose a "Boss" will no longer be visible.