How do I get this shade effect?

Link: Show off your UI designs - #107 by 7kayoh
In this link, when the 3 lines button is clicked, the gui on the left is shaded, like it becomes darker, how do I make this effect?

There are multiple ways of making it, get a frame and set it to Transparency 1, and when clicking the 3 lines, you can tween the transparency of the frame to match the time of opening the menu.