There are two macros that allow you to create a subject for a wiki space.
Popular Labels
Use the Popular Labels macro to create a list of the labels used most frequently in a wiki space.
https://confluence.atlassian.com/conf613/popular-labels-macro-964960897.html
List Labels
Use the Labels List macro to create an alphabetical list of the labels in a wiki space.
https://confluence.atlassian.com/conf613/labels-list-macro-964960851.html