Basically it's a function that converts an array of an arbitrary number of numbers into a probability distribution, such that the smallest number in the array will be converted to a number closest to 0, and the largest number in the array converted to a number closest to 1.
I was inspired to make this visualizer after watching this clip of an LLM explainer made by the legendary 3Blue1Brown. That particular clip introduces the concept of the softmax function.
You can also read this Wikipedia article if you're more of a text person.