The UserInput control is a textbox with a button, which opens the User library and lets the editor select an user.
After confirming the selection within the User library, the textbox will be set to the fullname of the selected user.
This control is specifically useful for the Author field, although it is not assigned as edit control by default.
The dropdown-combo control which is currently utilized as edit control on this field is not manageable when you have 1000 or more users.
To enable this control, add the ctfp cms:control to the Author field and specify the value userinput.
Specifying the ctfp cms:facetvalues, you can even filter the users shown within the library.
Filtering on enabled users only and also excluding visitors, specify "USERENABLED=0;ISVISITOR=0" as the value for this ctfp on the Author field.
(Notice that you need to specify 0 to only show enabled users, because the UserEnabled facet filters on the Disabled column.)