Depth passes contain information relating to the distances of objects in your scene, to the camera. Each pixel color in the depth pass defines this distance. Depth passes are commonly used to mimic depth of field blur, but have a variety of other uses.

In this tutorial, Foundry's Head of Creative Services - AMER, Terry Riyasat, explains the Depth Generator in NukeX and how it can be used to create a depth pass from footage for grading, masking, softening and more.