ERG mode is a setting where a device controls the resistance of the bike based on a preset target (such as 150w). This mode can be utilized to maintain a consistent output with changes in cadence. It's most commonly used to perform a prescribed workout that calls for power targets for each interval.
ERG mode can be engaged within an app (such as Trainer Road or Zwift, while using the workout feature) or by your head unit, if your head unit supports this function. This eliminates the need to shift to find the target for each step and allows you to focus on complete the workout accurately.