Debris

The Debris service allows scheduling guaranteed destruction of an object without yielding.

Advantages

Besides creating a bit of a mess, objects that are no longer required can use up system memory and cause an experience to run slower over time. For this reason, it's always advised to call Instance:Destroy() on objects you no longer need. In some cases, however, an object may have a specific period of utility before it can be destroyed.

Consider a wall being smashed into individual bricks. If you want a brick to linger for 3 seconds before being destroyed, you can use the following code:

task.wait(3)
brick:Destroy()

However, waiting causes the thread to yield which may be undesired. To avoid yielding, a callback function can be scheduled to run on a new thread after 3 seconds:

task.delay(3, function()
	brick:Destroy()
end)

Or in one line:

task.delay(3, brick.Destroy, brick)

While this now avoids yielding, it has a potential drawback in that the scheduled callback will never run if the script is disabled or destroyed before the callback runs.

This is where Debris has a specific advantage, as it does not yield the current thread and runs outside the context of the script, guaranteeing the instance is eventually destroyed even if the script is disabled or destroyed. The following code does not yield and guarantees the instance will be destroyed:

Debris:AddItem(brick, 3)

Note that Debris has a hardcoded maximum of 1,000 objects, so if more than 1,000 items are added, the oldest debris will be destroyed instantly to make room for new debris.

local Debris = game:GetService("Debris")

local ball = Instance.new("Part")
ball.Anchored = false
ball.Shape = Enum.PartType.Ball
ball.TopSurface = Enum.SurfaceType.Smooth
ball.BottomSurface = Enum.SurfaceType.Smooth
ball.Size = Vector3.new(1, 1, 1)

local RNG = Random.new()
local MAX_VELOCITY = 10

while true do
	local newBall = ball:Clone()
	newBall.BrickColor = BrickColor.random()
	newBall.CFrame = CFrame.new(0, 30, 0)
	newBall.Velocity =
		Vector3.new(RNG:NextNumber(-MAX_VELOCITY, MAX_VELOCITY), 0, RNG:NextNumber(-MAX_VELOCITY, MAX_VELOCITY))
	newBall.Parent = game.Workspace
	Debris:AddItem(newBall, 2)
	task.wait(0.1)
end