Could someone please define arrow "spine". I see a lot of talk about arrow spine, but can't figure out exactly what it is. Sorry for such a basic question.

An arrow with a lot of spine is meant for higher poundage because it does not flex as easy as lower spined arrows, which are meant for lower pundages.

Spine is the amount of flex in the arrow. The tip or broadhead is the resistance. When you release the string the arrow will flex slightly to overcome the resistance caused by the tip or broadhead. The heaver the tip, the more resistance or flex on the arrow. Some flex is required to in order to tune your setup. Match your arrow to the manufacturer's charts that take cam, weight, draw length, broadhead weight and type of release you are using to determine the correct spine for your setup. Match this and you will have few problem tuning your setup.
Good Hunting...