Imagine a freeway, with no traffic lights or cross traffic. Now imagine driveways (mostly for businesses) and local streets directly leading onto the highway. Since there is no cross traffic, you can only turn right to and from these access points. But unlike a true freeway, these connections are not high-speed by any stretch of the imagination.
New Jersey, specifically northern Jersey, is full of these. Examples include Route 4, Route 17, and Route 208. Rare examples exist in other states, including Route 1 north of Boston.
The term is not in common use, since it was coined by Chris Sampang on misc.transport.road and has not spread much outside m.t.r. But it gives a name to an otherwise unnamed concept (you can't call it a freeway because of unlimited access, but calling it an arterial doesn't give the whole story).