You might want to have a look at the presentation here: http://app.emaze.com/@AOFOWLLC/how-to-use-tags-in-bitsofmine?autoplay#1

In short, you can attach a tag to an item in the single item view. In order to be able to do so, please ensure that metadata is visible. If it isn’t please tap on the button in the upper right corner on the single item view. Then, tap on the (+) button next to the # symbol and start typing…

Once you have attached tags to items, you can view your tags by selecting “Tag View” from the app bar in the timeline.