I assume that you are asking about the reasonableness test that was established in Katz v. United States. In that case, the Supreme Court created a two-pronged test to determine whether a person had a reasonable expectation of privacy. If such an expectation existed, the person could be said to have been in a constitutionally protected zone.
The first prong of this test is subjective. It asks whether the person was acting as if they expected to have privacy. If, for example, a person leaves something in plain view on the seat of their car, they are not acting as if they expect it to be kept private.
The second prong of the test is objective. It asks whether the expectation of privacy was reasonable. Here, the court must decide whether a reasonable person would have expected that a certain place would be private and undisturbed. For example, courts have ruled that it is not reasonable for a person to put trash out on a curb and expect that its contents will remain private.