A prescriptive easement is an easement created from an open, adverse, and continuous use over a statutory period. If you have ever heard of adverse posession, it is essentially the same thing except that it creates an easement as opposed to someone acquiring the title to land.
In some states (such as MA) the law also requires that the continuous use be "exclusive", meaning that only the person claiming the easement by prescription has been using it during the statutory period, not just everyone who comes along.
Also, a prescriptive easement may be defeated if the landowner occasionally blocks the access to everyone. That way, anyone who incorrectly believes he or she has a right to go across the land will discover it is not true.