Can someone explain the math being done in this problem to me? Is it picking a random number between lower and upper?

Returns a random integer between min (included) and max (included).

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

the + 1 is for the max value to be included too. If you omit the + 1 the max will be excluded. Math.random() returns a floating point number between 0 and 1 (1 is excluded) then that floating point number is multiplied by the number of numbers between the two points. Then to get a number between the two points you will add your minimum point to the result. Math.floor will return the number as an integer that is less or equal to the result.

demonstrating with numbers for clarity. You want a random number between 10 and 20 where the number 20 is included.

Math.floor(
Math.random() * (20 - 10 + 1) + 10
)

so if we say Math.random() would return for example 0.34

let random = 0.34 * 11 // 11 is the number of numbers between 10 and 20(included). this will return 3.74
random = random + 10 // we add the min value to get a number between 10 and 20 (20 included). and we get 13.74
console.log(Math.floor(random)) // returns 13