A jump start is a colloquial term for a method of starting an automobile or other internal combustion engine-powered vehicle having a discharged battery.
Most U.S. passenger vehicles use a standard 12-volt DC battery which, when the driver turns the ignition key, briefly turns the crankshaft and powers the spark plugs until ignition is achieved and the engine can produce its own electrical power from its generator or alternator.
When a battery fails or is discharged, such as by inadvertently leaving one's headlights switched on while parked, the car's engine will not "turn over" when the ignition key is turned. Many motorists carry "jumper cables" which consist of a pair of heavy gauge wires with alligator clips at each end. An automobile with a good battery (owned, perhaps, by a Good Samaritan), is parked near the car needing the jump start and the cables are attached to each battery, polarity being maintained (the red wires are attached to the anode (positive) terminal of each car's battery, and the black wires are attached to the cathode (negative) terminals or to the engine blocks of the cars). The "good" car is started, and the dead battery allowed to charge for a few minutes. Then the other car can be started, the cables carefully detached, and the formerly stranded motorist goes on his or her way.
Many auto manufacturers specify methods of jump-starting their autos (e.g. the sequence of attaching or detaching jumper cables), for your safety please be sure and read your owner's manual!
In localities or in situations lacking in Good Samaritans, there is always the auto club, for its members. In mid-town Manhattan, New York City, standing next to a parked vehicle and holding up a set up jumper cables is likely to result in a taxi-cab stopping within a minute or two. The cab's driver is likely to sound surprised if the person with the cables needs to ask what the charge for the jump start will be. (In the late 1980s, the answer was $5.)
