ActiveScript refers to the LuaSourceContainer currently being edited by the user. If the user is not editing a script, this will be nil. Below is an example that shows how you can use this property to measure for how long a script was active.

local startTime = os.time()
local activeScript
local function onActiveScriptChanged(newActiveScript)
	if newActiveScript ~= activeScript then
		local deltaTime = os.time() - startTime
		print(("You edited %s for %d:%2.d"):format(activeScript.Name, math.floor(deltaTime / 60), deltaTime % 60))
	end
	startTime = os.time()
	activeScript = newActiveScript
end
game:GetService("StudioService"):GetPropertyChangedSignal("ActiveScript"):Connect(onActiveScriptChanged)