How to make a part disappear and appear again smoothly?

"the world's easiest question" but on to the question:

Using tweens to do this is cooler than this:

for i = 0, 1, 0.001 do
	script.Parent.Transparency = i
end

To use it, let's set up a TweenInfo for the tween settings.

local info = TweenInfo.new(
	Time = 5, -- Amount of time that it takes to finish the tween.
	EasingStyle = Enum.EasingStyle.Quad, -- The easing style, info can be found here: https://developer.roblox.com/en-us/api-reference/enum/EasingStyle
	EasingDirection = Enum.EasingDirection.Out, -- The easing direction, info can be found here: https://developer.roblox.com/en-us/api-reference/enum/EasingDirection
	repeatCount = 0, -- How many times to repeat, optional.
	reverses = false, -- Does it tween undo after tween, optional.
	delayTime = 0 -- Time to wait before tweening.
)

Now we'll set up a property table to change. It adds for certain property types like Vector3.

local propertyTable = {

	Transparency = 1

}

Time to tween!

local myPart -- The part to affect

local myTween = game:GetService("TweenService"):Create(myPart, propertyTable, info)

Now we can do alot of things, but we will only :Play it. To know more, look at this: https://developer.roblox.com/en-us/api-reference/class/Tween.

Full code:

local info = TweenInfo.new(
	5,
	Enum.EasingStyle.Quad,
	Enum.EasingDirection.Out,
	0,
	false,
	0
)

local propertyTable = {

	Transparency = 1

}

local myPart -- The part to affect

local myTween = game:GetService("TweenService"):Create(myPart, propertyTable, info)

myTween:Play()

That's it!

Links:

https://developer.roblox.com/en-us/api-reference/enum/EasingStyle https://developer.roblox.com/en-us/api-reference/enum/EasingDirection https://developer.roblox.com/en-us/api-reference/class/Tween https://developer.roblox.com/en-us/api-reference/function/TweenService/Create