A Wikidata identifier (Wikidata ID, Wikidata UID, QID, Q number) is the identifier used for a specific entry in the Wikidata database. This project encompasses entries for all sorts of things, numbering in the millions, each of which can be uniquely identified by its number in the system. These identifiers consist of the letter Q followed by a decimal number. And, yes, there's a Wikidata identifier for the concept of a "Wikidata identifier": Q43649390, which means that they got around to assigning identifiers to 43,649,389 other things before they finally self-identified this way.
These identifiers are capable of being used in many ways to identify things and express relationships between them. Among the proposed uses for them is this Unicode standards proposal which would allow custom emojis to be encoded by the Wikidata identifier of the thing depicted in the emoji, preceded and followed by escape-sequence delimiter characters.