Tracker fields can be imagined as columns in a tracker table. (While the Tracker items are rows data.)
See: Adding fields to a tracker.
For definition list of names values for the fields (in case you need to do some advanced magic with them), please refer to the dev site overview.
Some of the field types have some more documentation including examples, etc:
List via structure
Here is the source code for all the tracker field types: https://gitlab.com/tikiwiki/tiki/-/tree/master/lib/core/Tracker/Field
TrackerFields | Tracker Fields | TrackerField | Tracker Field | Tracker Field Type