Jumper cables are a tool used to start a car when its battery is dead. They connect the dead battery to a working battery in another car, allowing it to transfer power and jumpstart the dead car. Full definition
Getting a second set of jumper cables onto the recipient battery may be a bit of a squeeze if you've got good solid clips. (mechanics.stackexchange.com)
, or you could attach a supplemental battery with jumper cables while removing the main battery. (mechanics.stackexchange.com)
Also it made me laugh when you said you don't normally carry jumper cables in a car. (humblemechanic.com)