Can some on give me a detailed explanation of what "In Pairs" is and what it does?

Great question.

So when using a for loop you can either do

List = {1,2,3,4,5,6,7,8,9,10}

for i = 1, #List do --#List just gets the length of the list
    print(List[i])
end

this will print all of the values of the list by adding + 1 to i every time so it prints a different value every cycle.

however, if you do in pairs like so

List = {1,2,3,4,5,6,7,8,9,10}

for i, value in pairs(List) do
    print(value)
end

You will be able to actually just print value, instead of index through the List like the regular for loop.

These both will do the same thing but in some situations it is definitely better to choose one or the other.