What does this tag icon in Photos mean?

That does seem odd-- I don't remember that happening to me, but then I already attach a name keyword for each of the people in a picture (well, the ones I care about...) The keywords are more robust and reliable than the face IDs-- they last through system upgrades, and they export with picture files, for instance.

I have, more times than makes sense, mistakenly typed captions or titles into the Keyword field. Makes some pretty awkward keywords.

It means it the picture has keywords.

Go to View>Metadata and turn off keywords, and the tag will disappear. You can turn on and off other tags, as well.

When we export or share photos, the keywords may be included. Perhaps you have saved a photo to Photos, that has been shared with you and already included keywords.