An "egg timer" is a small device used to measure the time it takes to cook an egg. It helps ensure that the egg is cooked just the way you like it, by indicating when the time is up with a sound or by counting down the minutes. Full definition
I have bought a little 3 minute egg timer — concrete, visual — to give to my children when I go upstairs so that they know when I will be back. (fourprincessesandthecheese.com)
In reality they're most likely to be used for playing music (albeit, audio quality can be very poor) or as very expensive egg timers. (techcrunch.com)
I also bought a few egg timers, and I let the kids help me reset them to remind them to try again in a little bit. (twiniversity.com)