Nowadays, this is a hidden feature in a program that you can only access through some special sequence of keys and or mouse movements. If you know the secret, you'll get rewarded with a list of credits, a game, or who knows what. See https://www.eeggs.com/ for some of this.
Not too many Unix programs have Easter Eggs, maybe not any, though we probably should count programs like "make" that supposedly responded with "not war" to "make love" (according to "The Devil's Dictionary"). I've never seen a version of make that did that, though. If you know of a current Unix Easter Egg, let us know.
Years ago, "Easter Egging" was a not too friendly term applied to service techs who would replace parts pretty much at random trying to fix a problem. The less sense the replacement made, the more apt the tech would be labeled as doing this.
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More Articles by Tony Lawrence © 2011-07-06 Tony Lawrence
What happens then? Is there a ticker tape parade and heartfelt thanks from the computer it has reached? No, my friends, there is not. The poor packet is immediately gutted, stripped of its protective layers and tossed into the hungry maw of whatever application (mail, a webserver, whatever) it belongs to. (Tony Lawrence)