This field is a code fieldtype that gets saved as YAML instead of a string. Your input is validated on save to make sure you don't write invalid YAML.
The YAML field is one of the "catch-all" solutions for when there's no better way to work with an odd data structure. Recommended for developers only.
You really should know YAML if you're using this field, in which case you'll understand how the data is stored – exactly as written.
Refer to the YAML guide on how to work with data in general. We really can't be any more specific here. You understand, right?