The field id is the ID of your field, which is used to link to this field. The field id is always lowercase. The id of the field is set on the creation of a new field, using camel casing. You need the field id for the configuration of filters or display pattern.
The ID is automatically created from the name you insert:
Example: You insert the field name "My NeW fielDs #1 Purpose?
Automatic Id: myNeWFielDs1Purpose
You can open the Display Pattern Editor to see the JSON of a Content Item.