The depth of the resulting position in 3d. If you project from 3d into 2d, you are losing one dimension: the depth. So, if you do it the other way round, you have to reintroduce it or you would end up with an unlimited number of possibilities of points in 3d that match one point in 2d. That's what this parameter does. If you are viewing the resulting vector as a direction vector from the camera's position into world space (what you usually do when using this for picking), then this value doesn't matter. If you want to pick some point on a plane that you know the depth of, you can set it here to simplify your calculations.