I assume that you are asking about Justice Harlan’s formulation of the privacy test in his concurring opinion in the case of Katz v. United States. This was a case about the legality of a wiretap on a telephone booth. It was decided in 1967.
Harlan’s test has two prongs. Both of these are subjective. Taken together, they help us to determine when a search warrant is needed because a person is in a place that is private. The first prong of Harlan’s test has to do with the state of mind of the person whose actions are being observed. In order to have privacy, the person must act in a way that shows that they expect privacy. The second prong has to do with societal attitudes. The person must be in a place where society in general would typically think that the person should have privacy. It is not enough for the person to think they have privacy. The situation must also be one in which most people (not just the person in question) would expect to have privacy.
Thus, Harlan’s test in Katz is formulated with reference to the belief of the person whose actions are being observed and the beliefs of society as a whole.
In Katz v. US, Justice John Harlan wrote in his concurring opinion that authorities would have to get a warrant to monitor communication from any place where a person might have a "reasonable expectation of privacy." He was agreeing with, and slightly expanding, the majority ruling that a telephone booth was one such place, because the person in question had made an effort (shutting the door behind him) to ensure his privacy. This essentially meant that privacy was not simply something one could only enjoy in one's own home. Harlan reinforced the court's decision that electronic as well as physical violations of privacy required a warrant. He further ruled that "warrants are the general rule, to which the legitimate needs of law enforcement may demand specific exceptions," suggesting that while there would be many exceptions, the onus would be on law enforcement to demonstrate them if the Fourth Amendment protection against unreasonable searches and seizures was to have any real force. So Harlan strengthened and expanded the definition of privacy through the "reasonable expectation" test.