Understanding the functionality of a button with a rotating arrow in GNOME 3 status menu

There are no tool tips for buttons in GNOME Status menu. I wanted to know the function of the 2nd button from left at bottom in the following image:

It shows a lock sign when clicked.

OS: Ubuntu 18.04

Solution 1:

The 2nd button from left at bottom in the image you have attached is for "screen rotation lock" (landscape/portrait orientation).

Currently it is unlocked which means if you physically rotate your device screen by 90 degrees, your screen also should rotate accordingly (change orientation). When it is "locked", the screen won't change the orientation even if you physically rotate the display device (monitor).