What is string.sub? How to use it? The Roblox article doesn't quite make sense...

Could you please explain to me what is string.sub? It is pretty useful, as I saw some uses of it, like this one:

local Text = "Type in anything you want"

for i = 1, #Text do
    wait(0.05)
    script.Parent.Text = string.sub(Text, 1, i)
end