OffMesh Links are used to create paths crossing outside the walkable navigation mesh surface. For example, jumping over a ditch or a fence, or opening a door before walking through it, can be all described as OffMesh links.
We’re going to add an OffMesh link component to describe a jump from the upper platform to the ground.
Now you have a functioning OffMesh Link set up! If the path via the OffMesh link is shorter than via the NavMesh, the OffMesh link is used.
You can use any game object in the scene to hold the OffMesh link component, for example a fence prefab could contain the OffMesh link component. Similarly you can use any game object with a Transform as the start and end marker.
The NavMesh bake process can detect and create common jump-across and drop-down links automatically. Take a look at the Building OffMesh Links Automatically for more details.