A "Claim" covers your land and renders walking, building, demolishing and taking stuff on this land criminal actions. That means that players cannot just walk by and do what they want without leaving any trace, but that they would first have to actively make the decision to commit a crime. By committing a crime, they'd leave behind a "scent" which would allow rangers to track down the criminal. The worse a crime, the longer it's scent lasts.
While that culls away the curious and the cheeky visitors, it would still be no obstacle to anybody who isn't afraid of punishment. If you want to know more, check "Ranging" and "Scent" in the Haven Wiki.
To claim your land, you need the skill "Yeomanry", which requires "Will To Power" and "Farming". You can buy "Farming" after getting the skill "Plants Lore".
IRC is irc.synirc.net, #haven