Is it essential to understand this exercise in order to progress?

function getRandomInt(min, max) {
    return Math.floor(Math.random() * (max - min + 1)) + min;
}

When working with math functions, it's often best to start small. so let's say we want numbers in a range of 2 to 4. The value of (4 -2 + 1) is 3. You multiply 3 by a random value in the range of 0 to 0.9999999999999. Once you apply floor, you are left with either 0, 1, 2. Add the lower number to these and your possible values are 2, 3, 4.

This level of maths is pretty common. A lot of array handling, string work and some database stuff requires differences and displacement. but you can usually break the maths down, use sample values and experiment until you come up with the formula that works. But a lot of maths in computing is pretty simple once you've done it a couple hundred times.

The random number is in the range of 0 to 0.999999999999. If you want a value in 0-4 (ie. range is 4), then the smallest number you could get is 0*4 or 0. The biggest number is 0.999999999 * 4, which is 3.9999999999999 - which floors to 3. It's not big enough. So you increase the range by one. 5 time 0 is 0. the largest value is 5 * 0.9999999, which is 4.999999999. This floors to 4.

It's the same equation. In the first part, he wanted variation from 1 to the top limit. minimum was implicitly always 1.

Math.floor(Math.random() * (max - min + 1)) + min; //formula
Math.floor(Math.random() * (max - 1 + 1)) + 1;           //apply minimum of 1.
Math.floor(Math.random() * max) + 1;

Thank you Simon! You're explanation is brilliant. You cleared up the same confusion I had of Arikaturika Tumojenko.