Under Jewish law, on the Sabbath (Friday night to Saturday night), you’re not supposed to carry any of your possessions between private domains (like your home) and public domains (like outside your home). But what if you want to take your baby to synagogue? What if you want to carry your keys from inside your home to outside where your car is parked? You can’t do it. Jewish law forbids it.
But Orthodox Jews figured out a loophole. All they have to do is turn a “public” domain into a “private” one and problem solved! They accomplish this by creating an eruv (AY-roov).