Can anyone explain to me what :sub() and :len() does ?

so like here just a script can anyone explain to me what the :sub() part does. This is just a map changing script. I just don't understand what it does. same with :len()

-- Store all your maps in Lighting.

current_map = nil

names = {}

game.Players.PlayerAdded:connect(function(player)
	for i, v in pairs(names) do
		if v == player.Name then 
			player.Chatted:connect(function(chatmsg)
				if chatmsg:sub(1,4) == "map/" then
					local map = chatmsg:sub(5, chatmsg:len())
					if game.Lighting:FindFirstChild(map) then
						if current_map ~= nil then
							current_map.Parent = game.Lighting
						end
						game.Lighting[map].Parent = game.Workspace
						current_map = game.Workspace[map]
					end
				end
			end)
		end
	end
end)