The Flickering Screen
Modern MacBooks and iMacs are equipped with ambient light sensors. macOS uses these sensors to constantly monitor the brightness of the room you are in. If a cloud passes over the sun, or if you turn on a desk lamp, the operating system automatically and dynamically adjusts the brightness of your display to compensate. While this is intended to reduce eye strain, it can be incredibly irritating for creative professionals. If you are color-grading a photograph or editing a video, having the screen brightness constantly shifting in the background makes it impossible to accurately judge contrast and exposure levels.
Locking Your Luminance
To ensure a consistent visual baseline and stop macOS from automatically changing your display brightness, you must disable the auto-adjustment features in the Displays settings.
Click the Apple logo in the extreme top-left corner of your screen and select System Settings. In the left-hand navigation sidebar, scroll down and click on Displays. On the right-hand panel, locate the brightness slider. Immediately below that slider, you will see a checkbox explicitly named Automatically adjust brightness. If your screen has been shifting on its own, this box will be checked. Click the box to uncheck it. Additionally, if your Mac supports it, you may want to uncheck the True Tone box just below it, which stops the Mac from automatically shifting the color temperature (making it warmer or cooler) based on ambient light.