Signal detection theory, which at its most basic, states that the detection of a stimulus depends on both the intensity of the stimulus and the physical/psychological state of the individual. Basically, we notice things based on how strong they are and on how much we're paying attention.
For example, A hit is when the signal is present and detected. A miss is when the signal is present and not detected. A false alarm is when the signal is absent but is detected, like hearing your phone ring when no one is calling or a knock at the door when no one is there.
