Tags are just a way of grouping different posts together. For example, let’s say you have an interest in Chess and you happen to be reading a thread about Chess. Let’s say for example you are reading this thread:
If you go to that thread, you will note that one of the tags under the thread title is Chess. If you click on that tag, Discourse will bring up a list of all threads that are tagged with Chess, so it’s a quick and easy way to find a bunch of threads all on a particular subject that you are interested in.
Most of the tags, like the Chess tag, are specific to one forum, because that is where most of the posts for that topic well go. There are 68 threads tagged with Chess, and all but one of them is in the Game Room (the one oddball is in CS since it’s looking for a video).
Some tags span multiple forums, though. The ukraine-invasion tag is a good example. This has breaking news threads in MPSIMS, rants in the Pit, opinion threads in IMHO, etc. But if the war in the Ukraine is something that you are interested in, clicking on that tag will give you a list of all of the threads that are tagged with it.
There is a list of all tags here:
https://boards.straightdope.com/tags
Don’t worry about making more work for the mods.