Screen Flickering

Before assuming screen flicking is a hardware problem, go to Apple menu -> System Preferences (or System Settings on newer MacOSes) -> Displays

The color setting or display setting depending on OS version will also include the Hz (Hertz) of the display resolution. In certain lit environments, the synchronization of the display's own refresh rate will cause flickering to be more pronounced than others. Moving into a dark room temporarily can verify if the refresh rate is the problem. Overhead lights can especially pronounce the out of sync refresh rate of the computer. Adjusting the refresh rate can help. If you get to select the light bulbs in the lighting make sure the refresh rate is comparable to that of the computer. External displays can have this problem too. Some third party programs like EasyRes offer more finite control over the Hertz of the display frequency. SwitchResX and DisplayConfigX may also work.

Apple's notebooks (MacBook, MacBook Pro, and MacBook Air) may have additional hardware issues, which are discussed more below:

Flickering can frequently mean some issue with the video cable into the screen.

Notebook screens from Apple have been outfitted with a specific resistance when you push the screen back a certain amount. Don't attempt to push further back when you meet that resistance. A fall or drop can force a screen to go back further than it is designed to do, and start to pinch the cable to the screen. Newer OSes may have different tolerances for signal degradation.

AppleCare Plus has certain accident protection you can get, and you should read the documentation of whether it is available only at time of purchase or later. Official Apple Support

Apple Service Programs sometimes addresses manufacturer level defects discovered after the fact you can mail in to have serviced even after warranty.

Older Powerbook and iBook computers may have these and other issues as well, and there are certified hobby people who can look at these issues as well. Circuitry can get old and need to be replaced by someone with the right skills to do so. User Groups - Apple are a good place to start.