It is not the meaning of the : per se, but what is present, or rather absent before it.
The refspec format is
<+><source>:<destination>
(optional + for non-fast forward)
So when you do something like git push origin :featureA, you are specifying an empty source ref and basically making the destination “empty” or deleting it.
PS: Note that the refspec of : or nothing doesn’t mean push nothing to nothing however. It makes git to push “matching” branches: for every branch that exists on the local side, the remote side is updated if a branch of the same name already exists on the remote side.