What is this symbol?

	local door = script.Parent.Parent.Parent.Door:GetChildren()
	for x = 1, 56 do
		wait(0.01)
		for i = 1, #door do
			door[i].CFrame = door[i].CFrame * CFrame.new(0, .25, 0) 
		end
	end
end

What does # mean? O.o

local tble = {"lol", "lul", "lil"}

In the above, if we used "#tble" that means the number of things in that list. What you have there, is the number of objects inside of "door".