It's a function that converts an input array (of any length) of numbers into an output array, such that the smallest number in the input array will become the number closest to 0 in the output array, and the largest number in the input array will become the number closest to 1 in the output array. (that's a mouthful!)
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 the reading type.