Can anyone here explain to me what a "tag" request is?

We have a "like" page from our vacation home. We don't know who owns it, exactly, and we don't have any "friends" on our friend list from there, but we do post on the page now and then when conversations come up ...

Well, we put a picture of Oreo on the dock with a freighter behind her, when people were posting their pets.
That was a few months ago.

Today the daughter of one person we know a bit, sent us a
"tag" request for this picture. What does this mean? I can't message either her or her mother, since they are not on our friends list (we keep a very low profile).

I hate to refuse or ignore and offend them... a very small tight community.... but then can this backfire on us some way if we accept?

I called my son and he was very vague about it...etc.

Sorry to be so clueless!