A dataset is an identifiable collection of structured data objects (distribution list) unified by some criteria (authorship, subject, scope, spatial or temporal extent…) this is called metadata.
The harvest plan is the configuration used to import data into your catalog. https://github.com/GetDKAN/harvest/blob/master/schema/schema.json
Schema is a human and machine readable vocabulary that allows you to annotate a list of properties and validate JSON documents.
Metadata is structured information that describes, explains, locates, or otherwise makes it easier to retrieve, use, or manage a data resource. The challenge is to define and name standard metadata fields so that a data consumer has sufficient information to find, process and understand the described data. The more information that can be conveyed in a standardized regular format, the more valuable data becomes.
There are a number of specifications for dataset metadata. By default, DKAN ships with a "Project Open Data"-inspired schema for datasets.