digiKam makes it relatively easy to tag photos. Select one or several photos, expand the Caption/Tags right sidebar, tick the tags you want, and press the Apply button. You can also assign keyboard shortcuts to the often-used tags to speed up the tagging process. To assign a keyboard shortcut, right-click on the tag you want in the Caption/Tags right sidebar and choose Properties.
Press the Shortcut button, and hit the key combination of your choice to register the shortcut. Press the OK button, and you are done. Select now a photo, press the specified keyboard shortcut, and digiKam assigns the appropriate tag to the photo.