The Pokemon eggs can be hatched with incubators. All players, including you, have an incubator in their backpack that they can use to hatch eggs. Incubators are small, and thus have only 1 spot for an egg to be incubated.
The free incubator you get from the game can be used forever, all other kinds of incubators can only hold up to three times usage before they break.
Additional Incubators are purchased with ingame coins (earned through holding gyms or paying with real money). ( I strongly suggest you buy stuff through the shop, instead of through the ingame app. Buying items from their website shop gives you extra benefits. )
You can use up to 9 incubators at once, without the fear of incubating them individually. This allows you to perform more than 1 egg a day if you have more incubators. Super incubators ( The purple version ) even cut down the time for the egg by a 3rd of the total time. So, if you would have a 10KM egg, it would only need 6.7KM instead of the 10KM walking.
