It's just an old school way to check your alternator. You pull the positive battery cable off the battery while the jeep is running. If the engine dies, the alternator is bad, if the engine stays running, the alternator is good. It's just another easy check you can do to help rule out any problems with the alternator.
1987 wrangler, 258 I-6, 5-speed.
2007 liberty sport 4X4, automatic
2009 ford ranger 4x4 FX4 OFF/ROAD, 5-speed