New iOS developers may be unfamiliar with the Bundle Identifier (Bundle ID). You need to know it while submitting apps to the App Store or utilizing some third party APIs, such as Facebook and Google APIs.
While creating a new Xcode project, you need to enter a product name and an organization identifier in the dialog shown below. And these two items are joined together to form the bundle ID, such as com.mycompany.UIButton.
To get the bundle ID of an existing project, select the top project item in the project navigator at the left. Then select TARGETS -> General. Then the bundle identifier is shown as below:
Here you may change the bundle ID to any form you like for a project already created.