What does the term local do? I really want to know.

local sets the variable so that it's only available in the current scope. in these examples, world is just the name of the function, it could be anything

local a = "Hello,"

function world()
    local b= "world!"
end
world()

print(a,b) --> "Attempt to index a nil value (b)"

This happens because b is now only available inside of the function (The scope) where it was created.

The following works because the value of b is created outside of the function world(), so it can be accesed outside of it.

local a = "Hello,"
local b

function world()
    b= "world!"
end
world()

print(a,b) --> "Hello, world!"

This works too because b isn't local.

local a = "Hello,"

function world()
    b= "world!"
end
world()

print(a,b) --> "Hello, world!"

Another thing to note is that local variables are indexed a bit faster (barely noticable though)