While it literally refers to a cross, it has evolved to mean:
* The most important or critical part of something: This usage comes from the idea that the cross was the central point of the crucifixion, a pivotal moment in Christianity.
* The essential point: This meaning emphasizes the crucial element of a situation, argument, or problem.
For example, you might say: "The crux of the matter is that we need to find a way to increase funding." This indicates that finding more money is the most important factor in the situation.