If a constant's value can't change, why does a for in loop use a constant if the value of it is going to change?

I'm not sure what you mean here - the examples in this section use a var to store the array of items, and a for - in loop to iterate over the array, printing out each item. Where are you seeing a constant?

There are code examples in the Swift documentation like this:

let numberOfLegs = ["spider": 8, "ant": 6, "cat": 4]
for (animalName, legCount) in numberOfLegs {
    println("\(animalName)s have \(legCount) legs")
}

Where a let statement is used to create a static array of values before the for - in loop iterates over them, and while this is technically correct, a static array here works, in practice arrays in applications are more typically variable, and the more common implementation of this would be var numberOfLegs = ["spider"...]

This code could be modified as follows, to use a constant inside the loop like this:

let numberOfLegs = ["spider": 8, "ant": 6, "cat": 4]

for (animalName, legCount) in numberOfLegs {
    let printString = "\(animalName)s have \(legCount) legs"
    println(printString)
}

Here the the output would show the same: spiders have 8 legs cats have 4 legs ants have 6 legs

This works because of the way variables are scoped - each time the for - in loop iterates, the printString variable is destroyed and created fresh, so it's not a variable being reassigned, but a constant that only exists within the scope of each individual iteration.

I understand your confusion Erica DeJoannis, Amit does repeatedly say in the video that the x in for x is a constant. I think that what he means is this:

For each iteration of the array, x has one constant value - you cannot change the value of x inside of the for-in loop brackets. In other words, x acts as a constant, not a variable, inside each loop iteration.

After the loop iterates one time, it increments x, and x remains constant at this new value for the next iteration of the loop and cannot be changed.