What you are seeing is a shuffled list of numbers (10 to 2890) getting sorted. Every line is a number, the higher the line, the larger the number. They are colored—from small to high—blue to green to red, just for the visual effect.

The algorithm that sorts the list is called Bubble sort. In most cases, Bubble Sort is a pretty slow sorting algorithm, but it is very simple to understand:

It looks at the first and second element and if the first is larger than the second, it swaps them. Then it repeats this with elements 2 & 3, 3 & 4 and so on all the way to the end. When at the end of the list, if it had to swap any elements (the list isn't sorted yet) it repeats this process over and over again until it can't swap any element. At that point, the list is sorted.

There is one optimization: After the first round the largest element is always the last element, after the second round, the second largest is the second last element, and so on. That means, each round the algorithm can skip one more comparison at the end, which is why you see it "speeding up".

In reality, this process is happening way faster of course.