What does the 5th scene view tool do?

So i’m really new to programming, developing a game and everything so i decided to start out with unity!
I Watched the Tutorials and made some notes but i can’t seem to find the use of the fifth scene view tool after the scale button.
Anyone care to fill me in? The one that activates with the t key?

pls.

It’s the Rect tool.
https://docs.unity3d.com/Manual/UIBasicLayout.html