The cockpit is a busy place. A competent pilot is expected to be able to simultaneously and consistently control airspeed, altitude, and the direction of travel, while at the same time communicating with controllers over the radio, performing navigation duties, and scanning out the window for other aircraft in the area.
It is no wonder that student pilots are often overwhelmed in the beginning by the amount of multitasking that must be done, and also by the amount of information from different sources that they must process. A particularly common problem for many students is a tendency to focus on one instrument, for example the altimeter, while losing track of another instrument, say airspeed indicator. Another example is the student who tends to keep his eyes inside the plane scanning the instrument panel, and neglecting to look out the widow.
Problems like these are quite natural, but they must be overcome. Most instructors are trained to instantly notice when a students eyes stop moving, and are quick to bring it to the student's attention each and every time it happens.
To help students master this skill, flight Instructors teach students to establish a systematic scan, which involves panning the eyes from one side of the cockpit to the other in a set pattern. This forces the eyes to survey all the necessary items without spending too much time looking in any one place.
It may help students to realize that they do the same thing to a lesser scale when driving a car, where one also must constantly shift attention between the front window, the mirror, and the speedometer. It becomes easier over time, as the student learns to control the airplane more precisely.