Also known as: "bNodes"
Nodes in an RDF graph that have no URI label.
Blank nodes describe a thing without naming it. I can describe a person "brown hair, interested in bicycle racing, lives in China" without naming them, and often that description is useful. I can even attempt to identify a particular person - "the guy who wrote the book 100 Years of Solitude" - so that we can try to be sure if we are talking about the same person.
Uniquely Identifying bNodes with nodeID
You can identify BlankNodes locally with a NodeId. that ID can be used to talk about the node inside your particular file/store of information, but you can't use it to ID the node externally.
This can be convenient because the NodeId can be a short, human readable, name. Whereas assigning a full URI involves some work.
Uniquely Identifying bNodes by InverseFunctionProperty
BlankNodes can be uniquely identified if they make use of a predicate (SubjectPredicateObject) that has the "InverseFunctionalProperty." That means that the predicate and the object are guaranteed to uniquely identify the subject (the blank node.)