what does it mean

when a dog mouths another dog on the neck?

for my pups it's just playing. They do it all the time and then they come loving up on me and share their slobbery necks with me. GROSS

It's play here ..... also mouthing muzzles.

If they are playing around when this happens....then I would say it is play. If they seem serious, it could be a domination technique of sorts.

My dogs do that when they're playing. I've also seen dogs do that while they are fighting.
While some male dogs are humping another dog they try to hold the female down my holding her with his legs and trying to pin her down with his mouth. Sorry if that's TMI.

Jager was playing with a Great Dane earlier today, and they kept doing it to each other. Neither dog seemed to mind, tails were wagging the whole time, i just wondered if they were fighting for dominance or something.

My dogs do this occasionally! Lol.... With them it is all in play.... but the same techniques can lead to a dog fight with other dogs....
I guess it is best to learn how to read your dogs..........