Difference between XML_TAG() and XML_CLOSE_TAG() in VASP

In xml.F file, these two tags are defined.

The comment sections given for these subroutines is as follow.

!=======================================================================
!
!  subroutine XML_TAG (TAG)
!    writes a single TAG to the xml
!    and pops the tag to a stack
!
!  subroutine XML_CLOSE_TAG
!    pop a TAG from the stack and write it to the xml file
!
!=======================================================================

I don't understand exactly what these means. Can somebody explain in the comment section?