What does it means when someone "asked to tag your photo"on facebook?

[sabrina]

Yeah, that's pretty much it.
I.E. "John Smith asked to tah your photo"
What does it means??

[Ash]

Facebook, and several other systems, have facial recognition so that they can detect faces in pictures and then allow people to "tag" someone in the photo. Basically someone picked you out in the photo and assigned your name to it.

It sounds like you have the security feature enabled so that it asks your permission before posting the tagged photo to your timeline.

[niladri]

this is nothing but, John Smith loved that photo and he wants to tag his friends to your picture.

If you don't like it, you may go to fb settings and then change it according to your need in the tagging tab.

[pri✍]

It might be just being polite, because tagging your pic means their friends now able to see it, eventhough the original Audience Setting you set is Friends (means no friends of friends will be able to see it).
This is mainly if that friend who asked you directly, say in the comment of that pic.

It might also because you set "Review tags people add to your own posts before the tags appear on Facebook?" to On
in Timeline & Tagging Settings.
The latter seems that caused the phrase you mention in your question.