If you have done some iOS development (or application development in general) already, the following example might seem familiar to you.
You are planning to build an app. You start collecting features, drawing some sketches, or your project manager hands the requirements to you. At some point, you start coding. After you have set up the project, you start implementing the required features of the app.
Let's say an app is an input form, and the values the user puts in have to be validated before the data can be sent to the server. The validation checks, for example, whether the email address looks like it's supposed to and the phone number has a valid format. You implement the form and check whether ...