Licking and chewing?

So I was grooming this mare at my barn and when I finished I went over to her head and started to rub/massage it gently. She stayed still and seemed to greatly enjoy it. She then lowered her head a few inches, making it easier for me to rub her forehead, and then... she started to lick and chew!
This has happened to me before with another horse while rubbing and massaging her, but this time I was doing it on her neck and near her ears. She actually lowered her head rather low, blew out, did what sounded like a small nicker or something, and a bit later she started to lick and chew...

I know this is a good sign but what does it mean? I am really curious to know! I know when you do join up it's the sign they accept you as a leader and want to come in with you, but I don't think it means quite exactly that in this circumstance.

Thanks!