Pith is the central idea or essence of something. If you’re in danger, you could exclaim, “I would greatly appreciate it if someone would provide assistance.” Or, you could get right to the pith of your point by shouting, “Help!”
In botanical terminology, pith refers to a spongy, central cylinder of tissue found inside the stems of most flowering plants. If you know a lot about plants, that fact should help you remember the primary definition of pith. If not, just take our word for it. You can think about the pith of an argument like the pith of a plant: both lie at the heart or core of something.