Many of the variables that psychologists are interested in are abstract concepts such as aggression or intelligence. Operationalisation refers to the process of making variables physically measurable or testable. This is done in psychology by recording some aspect of observable behaviour that is assumed to be indicative of the variable under consideration. For example: Aggression-a psychologist may record the number of punches thrown. Intelligence-a psychologist may record the number of puzzles solved in an hour, or calculate the score on an IQ test.
Reification (regarding hypothetical variables like intelligence as having a real physical existence) is a danger, however.