When you create posts in many spaces, you will start to see the options of adding labels to posts with a handful of labels predefined.
This has been the case for a while, but some work has been done to make labels more prominent (e.g. adding them to the sidebar in spaces, increasing visibility in the post editor).
Note that General Topics does not currently have labels defined, but many of the other spaces do.
One benefit of Labels (versus tags, which are similar) is they can be subscribed to (tags cannot).
One downside is they are specific to a given space.
Labels are not currently required to post in public spaces.
It is possible this will be required in some spaces at some point in the future.
Regardless, labeling posts where applicable will help us help you more quickly.
Suggestions for additional labels in various spaces are, of course, welcome.