NavNesh Areas can affect NavNesh generation and agent behavior/pathing.

Obstacles

Areas allow to block of certain areas of the NavNesh both in editor and at runtime.

Obstacles can be created by creating a NavMesh Area Component, enabling the IsBlocking flag and linking it to a collider component.
In most scenarios, the linked collider should be a trigger, so it still allows physics objects to pass through, while blocking NavMesh generation.

• Static obstacles are basically free.
• Moving obstacles are a bit more expensive, but you should be able to have at least a couple of dozens of them.

Examples