An “override” (also sometimes called an overwrite) is a commission paid on the sales someone else makes. For example, you may have a sales person with a 5% commission (earns 5% of the sales value of whatever they sell). This person may have a sale manager with 6 direct reports, and may receive as his/her compensation 1% of the sales of all the people reporting to him/her. The 1% to the manager is an override. It is a common sales compensation mechanic in small or early stage businesses.
Over time, most businesses benefit from moving their sales leaders to a goal-based plan with payout thresholds (e.g., no variable pay earned under 75% of the year-to-date goal), and acceleration for over-goal performance.