You can assign one or more tags to selected photos. A tag can be one or more words that you want to associate with those photos.
To add new tags to photos, select the photos you would like to tag, then do any of the following:
Choose Tags ▸ Add Tags.
Drag the selected photos and drop them on the desired tag.
When you use Ctrl+T or Tags ▸ Add Tags you can type in the names of one or more tags, separated by commas. Once you have created a tag, you can rename it by selecting that tag in the sidebar and choosing Tags ▸ Rename Tag "[name]", by rightclicking on it and choose Rename... or double-click on the tag in the sidebar.
To change which tags are associated with a particular photo, select that photo, choose Tags ▸ Modify Tags or right-click on a photo and select Modify Tags and edit the comma separated list. To remove a tag from one or more photos, first select that tag in the sidebar, then select the photos you would like to remove, and choose Tags ▸ Remove Tag "[name]" from Photos or right-click on the photos an select Remove Tag "[name]" from Photos.
To delete a tag entirely, select that tag in the sidebar and choose Tags ▸ Delete Tag "[name]" or by right-click and select Delete Tag "[name]".
When you create a tag, it will appear in the sidebar under the Tags item, which is hidden if there are no tags. Photos can have multiple tags attached to them, and when you click on the name of a given tag in the sidebar, you will see all the photos associated with that tag.