The field path is the hierarchy of the field, which is the path where you can find the field. For the configuration of filters or display patterns, you need to access the value via field id or field path. The field path always starts in lowercase.
You can open schemas > indexing. There you find the field path of every field of the schema. You can copy it by clicking on the value.
Visual representation of the hierarchy of the field path syntax
You can open the Display Pattern Editor to see the JSON of a Content Item.