Hi Alicia , thanks for your question! In our community we haven't enabled tags, which is why you don't see the option when you add your post here.
To try out using tags, check out our demo community, we have tags enabled there. Feel free to add a post: https://sense7.forumbee.com/.
When searching, it will return tags that match your search phrase as shown here. Clicking a tag will show all posts with that tag.
When in a category you can also filter posts by tags:
I hope that helps! Let me know if I can clarify anything.