In the Info window, click the Add a Keyword field (or the field where other keywords appear if you’ve already added some), then type a keyword and press Return to add it to the photos.
As you type, Photos suggests keywords that you’ve used before. To delete a keyword, select the keyword and press Delete.
When you’re done, close the Info window.
You can use the Keyword Manager to assign keywords to selected photos, manage your list of available keywords and create a “quick-pick” list of your most common keywords so you can assign them more quickly.
In the Photos app Find and identify photos of people using Photos on Mac