These signs usually mean nothing, as trespass is a civil offence and you can only be prosecuted for a criminal offence. If you trespass (i.e. enter someone's land without permission) the landowner can ask you to leave and use reasonable force to make you leave. The law does not define "reasonable force"; it depends on the circumstances of each case. Recent law, passed to curtail the activities of motorway protesters, has created the criminal offence of aggravated trespass.