Repeat only executes once, why and how?

Not sure where your Lua experience stands, so ill explain absolutely everything.

TweenService works by creating a tween instance that contains all your movement data. It has 3 properties: the object you want to tween (this can be a 3d object, GUI or just any instance with numerical properties), a TweenInfo instance (this contains the information about your tween), and finally the parameters of the object you want to manipulate. TweenInfo has a number of properties: the duration of the tween (in seconds), the interpolation method (Ex. Linear, Quadratic, Exponential, Back, etc.), the directions it should apply the interpolation (should it interpolate on the start to end, just on the start, etc.) and a couple more that you wont really need to worry about.

With that out of the way lets create a tween. For this I will be tweening the instance Part for one second with the interpolation method set to Circular, interpolating only on the way in, to a new position of 1, 0, 0.

Ex.

local Tween = TweenService:Create(Part, TweenInfo.new(1, Enum.EasingStyle.Circular, Enum.EasingDirection.In), {position = Vector3.new(1, 0, 0)})

Note: Enums are just dictionaries for numbers, Enum.EasingStyle.Circular is actually just the number 9 but it makes it easier to remember what 9 actually represents when its written like this.

After the tween is set up you need to actually run it. This step is pretty easy.

Ex.

local Tween = TweenService:Create(Part, TweenInfo.new(1, Enum.EasingStyle.Circular, Enum.EasingDirection.In), {position = Vector3.new(1, 0, 0)})

Tween:Play() -- Execute the tween

lastly Tweens can be canceled or paused with :Cancel() and :Pause()

Ex.

local Tween = TweenService:Create(Part, TweenInfo.new(6, Enum.EasingStyle.Circular, Enum.EasingDirection.In), {position = Vector3.new(1, 0, 0)})

Tween:Play()

wait(2)

Tween:Pause() -- Pause the tween

wait(2)

Tween:Play() -- Resume the tween

wait(2)

Tween:Cancel() -- Cancel the tween

Make sure to set the time and interpolation method to whatever you want.

Addendum: "How would I go about when a person puts their mouse outside of the button in the midst of a tween?"

local EnterTween = TweenService:Create(Part, TweenInfo.new(1, Enum.EasingStyle.Circular, Enum.EasingDirection.In), {position = Vector3.new(1, 0, 0)})

local ExitTween = TweenService:Create(Part, TweenInfo.new(1, Enum.EasingStyle.Circular, Enum.EasingDirection.In), {position = Vector3.new(0, 0, 0)})

GUI.Button.MouseEnter:Connect(function() -- This is just a different way to set up functions, it works the exact same as your method, I'm just more accustomed to using this. So its not required for this to work.
	ExitTween:Cancel()
	EnterTween:Play()
end)

GUI.Button.MouseLeave:Connect(function()
	EnterTween:Cancel()
	ExitTween:Play()
end)

See if that fixes the issue.