the first line you asked about simply prints those values to the display. "?" is "newline" as documented in the manual.
$users.filter="User",""
is more interesting. The filter property requires an array. KiXtart will let you build a an array by setting comma separated values to a variable. In this case an array is created where "User" is the first element and an empty string is the second. the filter uses the "User" value.
