Blinking Light

Change a part twice in a row and you only ever see the second change: the script runs both lines in an instant. task.wait(seconds) pauses the script in between, while the rest of the game keeps running:

print("Ready")
task.wait(3)
print("Go!")

Go! appears 3 seconds after Ready.

To repeat something forever, use a while true do loop. It runs its body, then starts over:

local sign = script.Parent
while true do
	sign.Color = Color3.fromRGB(255, 0, 0)
	task.wait(0.5)
	sign.Color = Color3.fromRGB(0, 0, 255)
	task.wait(0.5)
end

This sign flashes red and blue every half second. A while true loop must contain a task.wait: without one it never lets the game continue, and Roblox stops the script with a timeout error.