Photos can store a list of keywords as part of their IPTC metadata. You can add categories to keywords to make them available to other programs, and to allow photos to be indexed by Spotlight.
In the Categories preferences, you can choose whether to add categories to keywords automatically. You can also add them manually by choosing Photos ▶ Keywords ▶ Add Categories to Keywords.
Once you have added categories to the keywords of photos, you can use Spotlight to search your photos by content. You can also create smart folders from categories and searches.
When you add categories to keywords, Shoebox will make sure that each photo has the correct file extension. If necessary, a hidden “.jpg” extension will be added to photos. Otherwise, Spotlight will not index them as images.
You may have added keywords to photos using other programs, such as Adobe Photoshop. To import keywords, choose Photos ▶ Keywords ▶ Get Categories from Keywords. Photos will be added to existing categories if possible; otherwise new categories will be created in /Imported/Keywords.
You can view a photo's keywords in the Finder by choosing Get Info. Keywords and other metadata are displayed in the “More Info” section.