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 and manage your list of keywords.
In the Photos app
Drag keywords into the Quick Group area of the Keyword Manager.
Photos gives each keyword in your quick-pick list a keyboard shortcut, so you can quickly assign the keyword to a photo.
Keywords that you add to the Quick Group area also appear in the Filter By pop-up menu. You can then choose a keyword in the menu to show items with that keyword. See Find and identify photos of people in Photos on Mac