An appeal in baseball occurs when the defense team observes an infraction during play which would have been otherwise allowed without the appeal. Appealing gives the defense a way to raise concerns about unlawful play taking place that the umpires may have missed.
An appeal can be made for multiple reasons, but the most common type of appeal revolves around the offense team batting out of turn. An appeal can be made on a batter or base runner providing their play has been infracted in some way.
When casting an appeal, the defense team needs to do so before the beginning of the next pitch or before their team has left fair territory.