As you move through the world, and collect items from Pokéstops you might just luck out and pick up an egg. You can incubate eggs in order to hatch them, which will net you a Pokémon in the progress. You start the game with a single incubator, although you can purchase more. The only way to get eggs though is by finding them at Pokéstops. To hatch an egg, you must first place it in an incubator. When you place an egg into the incubator, it will let you know how far you need to physically walk in order to hatch it. Initially, you'll only have access to a single incubator which lasts forever. You can purchase more incubators from the store for 180 coins each, and those will last for 5 uses before breaking down. When Eggs hatch, you can generally expect to find Pokémon similar to common Pokémon from the Pokéstop where you found the egg but may occasionally find something you have not otherwise seen.