Something is double if it has two parts or if there are two versions of it. If you order a double dip ice cream cone, there will be two scoops of ice cream for you to lick.
If you fall while you're roller skating and end up with a double arm fracture, it means you broke your arm in two places. A joke with a double meaning has two of them, and if you double your tips at the coffee shop by smiling more at customers, you're making twice as much money. In addition to the adjective and verb versions of double, there's the noun: your double is someone who looks exactly like you.