Items within the Gallery can be tagged for the purposes of filtering through them more easily.
This is particularly useful when a gallery has a great number of media files contained within it.
Select the Tags tab on the left to display all current Tags. There are a number of System Tags that will already be there as default.
You can click Add to create and name your own custom tag. Once you’ve saved it, you can highlight items in the gallery and click Tag/Untag to choose system/custom tags to associate with them.
You can filter by system/custom tags to find specifically associated files by highlighting the desired tag(s) in the Tag panel on the left.