Fields such as the ones pictured below (e.g. shirt size, primary position, doctor, doctor's phone number, etc.) are known as custom fields.

You can edit custom fields on the admin site by navigating to Setup->Registration->Registration Setup Wizard. You can add fields to either the parent page step or the player step. Choose which role you would like to add the field to, click the "New Field" button, and fill out the fields in the window.

Don't forget to click add field when finished.