Here's what we know about it so far. I'm primarily posting this for a summary explanation and anything else people want to add to it. I've gathered what I saw most frequently from the patch notes here.
What is a Visitor buff?
A visitor buff prevents a character from doing any actions that would leave scents while on the associated claim.
How did I get it?
Any time a player crosses through an open gate on any claim, they are given the buff. One buff per claim. If you enter a village through a gate, you get a buff for that village. If you then visit someone in that village on their personal claim, you get another buff if you pass through a gate for that personal claim.
But I'm friends with Joe Schmoe. How can I help him build stuff and work together?
As I mentioned the buff only prevents you from leaving scents. If you are a kin and have been given permissions, or have permissions from anything else (such as being in a party), you don't leave scents, and so can go about your intended business without being blocked by the buff.
I entered through someone's gates, but he locked me inside. Now what?
You're stuck and you'll have to hearth home. I guess you shouldn't enter random claims or follow people you don't trust.
I was attacked by raiders and I'm on the run. Will running through my open gates protect me?
If you flee through your gates with the engagement ongoing, the pursuer isn't blocked from continuing the fight. The crime scents are left when you are attacked. New scents aren't left until the engagement is broken and someone attacks you again.
Note that I've read at least one report at this time that the buff icon does not go away when you leave the claim, but that the buff is no longer affecting the character.
I'm not sure how murder scents are left, so didn't want to add that in to my last question and make any false assumption that running into your claim while being pursued might protect their life at the cost of some HHP.