An Egg is an Item in Pokemon Go, which contains a random Pokemon. Eggs can be carried around by a player and will eventually hatch into a new Pokemon after a certain distance has been traveled.Each Egg has a progress bar, which shows the total distance walked and the remaining distance still needed.
Along with Pokeballs and Potions, Eggs can also be found by interacting with Pokestops.
After walking 1 Kilometer (KM) (A little less than 2/3 of a Mile) this player was able to hatch an Egg. The Pokemon turned out to be a Seel!