Campaigns are for a run of posts with a start and an end. Tags are for everything else you might want to group by: the topic, the audience, whose idea it was, whether it is evergreen.
A post can carry as many as you like, and each one keeps its colour wherever the post shows up, so a board or a calendar tells you what a post is about before you read it.
Applying them
Open the tags picker from the composer and search what you have, or type something new and pick a colour for it. Nothing needs setting up first: a tag you invent while writing a post is a tag from then on.
Managing them
Settings, Tags is the other half. Everything the workspace has ever used is listed there, and it is where you rename one, change its colour, or delete it for good. Renaming updates every post carrying it, so fixing a typo does not leave two versions of the same label behind.
Finding things again
Once posts are tagged you can filter the schedule and planner down to one, which is the quickest way to answer "what have we said about pricing lately" without scrolling.