A very common kind of property is an identifier. Identifiers are unique, standardized ways of referring to specific items from other databases that explicitly represent specific items: names, organizations, concepts, etc. You may be familiar with Virtual International Authority File (VIAF), Library of Congress Authority ID, or Freebase ID as common identifiers. These are important because you can link several IDs to the same Wikidata item to associate people, concepts, and things across different databases or catalogs. (Wikidata has more than 700 authority control identifiers as of 2019.)
Identifiers can also include accession numbers, stable URIs (Uniform Resource Identifier encompass URLs and refer to a specific things instead of just web addresses), authority control, and catalog identifiers.