Focus in sport psychology refers to what is being paid attention to, or more specifically what exact task is the brain being used to complete at any given moment. This is ones “focus”. Usually focus is measured by either questionnaire or more accurately by where a persons eyes look when they are completing tasks. Visual focus is a strong indicator of mental focus.
Amateur athletes typically spend more time looking at and thinking about basic functions necessary to complete a movement. For example in soccer, an amateur will look at the ball when they are dribbling, shooting, or receiving a pass. Expert athletes tend to focus more on strategic elements of play due to their mastery of the basic mechanical movements. Again in soccer an expert would look at the open spaces available for their dribble, the target of their shot, or the options available for the ball while receiving a pass.
Two things tend to happen to focus when under the stress of a performance situation. The most common is a narrowing of focus, when an expert starts to ignore wider strategic elements and tunnel vision on a particular skill execution or play. This is the typical state that amateur players perform in most of the time when executing skills due to the necessity of focusing on the performance of the skill. The second is a broadening of focus, when a player does not focus enough on skill execution but is distracted by strategic concerns and thus makes an error.
Put together these two styles of losing focus account for most of the mistakes that can occur when under performance stress. Therefore a lot of interventions that help you handle or prepare for the in-game situation tend to improve your focus and reduce errors in play. Focus is also greatly affected by sleep, nutrition, and drugs (such as stimulants and depressants).