Tags allow you to add keywords to a conversation. You can create a tag by placing the "#" symbol before the word.
For example, writing this:
will generate this:
If you click on the tag icon in the message field, you can find the full list of tags available in your sphere. If you click on a tag, it will be attached to your message.
You can also find a message containing a tag:
By clicking on the same tag in a message
By using the search, and selecting the tag you are looking for
This is the result: