The maximum amount of nodes processed each frame in the asynchronous pathfinding process.
The pathfinder expands only a certain amount of nodes (NavMesh polygons) each frame while doing path finding. This allows the game to run smoothly without hiccups when long paths or large number of request are being processed at the same time, the flip side is that it can take number of frames for a path request to complete.
The iteration count only affects asyncronous pathfinding which used when setting NavMesh Agent destination using NavMeshAgent.SetDestination or NavMeshAgent.destination.
Increasing this value will cause the paths to be processed faster, but can cause some hiccups in frame rate. Default value is 100, a good range for tuning is between 50 and 500.