== returns a boolean

The == means equal in lua and it returns boolean, true or false based off of the condition, for example:

if (5 == 5) then warn('5 is 5') end

if (5 == 4) then warn('5 is not 4') end

if (true) then warn('true is true') end

if (false) then warn('false no no no') end

if (nil) then warn('nil is not valid') end

> Output:
--> 5 is 5
--> true is true

it is not setting CFrame of the HumanoidRootPart to the CFrame but returns if the part's CFrame is CFrame.new(PositionFirst.Position.X.... To fix that all you need to do is remove the == and return.

delay(1, function()
	local PositionSecond = HumanoidRootPart.Position

 	if (PositionSecond - PositionFirst).Magnitude >= 140 then
		HumanoidRootPart.CFrame = CFrame.new(PositionFirst.Position.X, PositionFirst.Position.Y, PositionFirst.Position.X)
	end
end)

Did you use return to return new character's CFrame? If so then just return his new CFrame like this

	if (PositionSecond - PositionFirst).Magnitude >= 140 then
		HumanoidRootPart.CFrame = CFrame.new(PositionFirst.Position.X, PositionFirst.Position.Y, PositionFirst.Position.X)
		return HumanoidRootPart.CFrame
	end
end)

Edit

Okay i see now, both of variables FirstPosition and SecondPosition are Vector3 values... not a problem, you check the magnitude and everything works well but the problem comes when you are setting HumanoidRootPart's CFrame to Vector3, doing CFrame with 3 arguments is same as setting it's position to Vector3 which won't move the Character, all you have to do is change FirstPosition and SecondPosition to HumanoidRootPart.CFrame and then setting his CFrame to one of it's positions. To get the Magnitude you would use CFrame.Position which returns Position as a Vector3.

local PositionFirst = HumanoidRootPart.CFrame

delay(1, function()
	local PositionSecond = HumanoidRootPart.CFrame

 	if (PositionSecond.Position - PositionFirst.Position).Magnitude >= 140 then
		HumanoidRootPart.CFrame = PositionFirst
	end
end)