Like its spiritual predecessors iPhoto and Aperture, Photos for Mac supports keywords (or tags). They’re an alternative way to organize photos without cluttering your albums. The iPhone and iPad versions support keywords too, but only in search; you can’t apply them, at least not yet, as far as I can tell.
When viewing any individual photo, click the (i) button in the right of the top toolbar (Command + i works too). A small tool window appears where you can edit various information about the photo—click the Keyword field and start applying them. Bonus tip: you can create multi-word keywords. Type a few words, then press Return to create the keyword.
Alternatively, you can click Window > Keyword Manager. With that tool window, you can select multiple photos and apply keywords to them all with a click. It’s pretty handy.
Later, on both macOS and iOS, you can search for keywords to focus on certain photos. It’s a powerful system, and I hope iOS allows us to create and apply keywords soon.