These are data elements that appear as data elements on standard templates. A typical example is the user’s email address and telephone extension on a letterhead.
In order to save a user typing their own details into the data capture UI every time they run a template, the Standard Data Elements feature allows them to enter these details into the configuration of a folder.
The values of the Standard Items are filled into any data capture UI in which the data elements with those names appear. At that point they can be left as is in the data capture UI, which is normally the case, or the user can change them for any specific template that is run.
See merging configurations for the exact mechanism of how this works.