Explain Like I'm Five

If you ever have the chance to use an old Apple II computer, run a text mode program, wait til the owner is looking in the other direction and turn the power off and back on quickly.

For about a second, before you hear the loud BOOP and the screen clears, you'll see whatever was on the screen just before you powered it off. But a few characters will be corrupted. Try it again, and wait a half a second longer than before. More characters will be corrupted.

For that brief second you're looking at the contents of the video RAM, then the ROM (Apple called what we call BIOS now "ROM") clears the contents and puts up the familiar text banner. The longer the power stays off, the more the contents of those flash cells decay and flip the bits that determine what character shows up in a location on the screen.