H-Piles are dimensionally square structural beams that are driven in the ground for deep foundation applications. Most soils at or near surface do not have the mechanical properties to support large structures. As you go deeper into the earth, there are layers (bearing strata) capable of doing so. H-Piles are manufactured and designed to transfer structural loads to these good bearing soils.
Sometimes, H-Piles are also used in conjunction with sheet piles to add lateral stiffness and bending resistance where loads exceed the capacity of sheet piles alone.