Any object in the PDF file may be labelled as an indirect object. This gives the object a unique identifier which other objects can use to refer to it.

The object identifier consists of a positive integer object number followed by a non-negative generation number. These two number must uniquely identify the object.

4 0 obj
(string object here)
endobj

The object may be referred to elsewhere in the file by the object number and generation number followed by the keyword R.

4 0 R

refers to the object defined above.