ShrinkWrap constraint pulls the owner towards the target's mesh.
This constraint is useful to make floor & wall, and can be used to constrain foot's IK target when make walking animation.
Check the preview image below:
Here we explain each options in the ShrinkWrap editor;
(ShrinkWrap contraint using Project method)
This method will cast a ray from pivot of owner along specified direction, if it hits the target's mesh, the hit point will be taken as the new position of owner;