What is the MacBook notch?
The notch is the cutout at the top centre of some MacBook screens. Apple calls it a camera housing built into the screen bezel. It holds the camera and sensors, and it sits where the menu bar runs across the top of the display.
Why it matters
macOS keeps menu items out of the notch, so the menu bar is split into a left half and a right half. A crowded menu bar can push some status icons behind the notch, where you cannot see or click them. Apple says the notch applies to MacBook Air and MacBook Pro models with a Liquid Retina or Liquid Retina XDR display.
The notch is fixed hardware. Software cannot remove it or move it. What software can do is draw around it, and that is what a notch app does: it places a small panel in the same spot so the space is useful.
Macs without a notch
Many Macs have no notch, including desktop Macs and older MacBooks. Some notch apps, NotchStar among them, draw a small floating pill at the top centre of the screen instead. See notch apps for Macs without a notch.
Read more
The full explanation, with a table of fixes, is in the MacBook notch explained. For hidden icons, see menu bar icons hidden by the notch. Developers read the notch size from the safe area.