SOIl NAILING is a method of slope stabilization (used e.g. for road cuts/embankments, foundation pit slopes etc.)
See these pages:
You basically drive steel bars into the slope face in a certain horizontall and vertical spacing (or insert into previously drilled boreholes filled with cement grout). Moreover, the face of the slope is normally further reinforced with a steel mesh, geo-grid or textile or shotcrete (sprayed concrete). It is cheaper than regular steel grouted anchors (prestressed) because the steel rods are common reinforcement bars for RC concrete structures, also the equipment needed is quite common.
I hope this helps,