In footwear, a hobnail is a short nail with a thick head used to increase the durability of bootsoles.
Hobnailed boots (known in Scotland as "tackety boots") are boots with hobnails (nails inserted into the soles of the boots), usually installed in a regular pattern, over the sole. They also usually have an iron horseshoe-shaped insert, called a heel iron, to strengthen the heel, and an iron toe-piece. The hobnails project below the sole and provide traction on soft or rocky ground and snow, but they tend to slide on smooth hard surfaces. They may also have steel toecaps.
Rotting in foul conditions in trenches: cured by chrome tanning rather than only using vegetable tanning.
Cold conducting through the hobnails into the feet: that, and need for strength, was cured by three thicknesses of leather in the soles.
Sole wear: toe and heel irons as well as hobnails.
Hobnailed boots were formerly much used for mountaineering to grip on sloping rock surfaces. Mountaineering hobnailed boots tended to also have large hobnails fastened to the extreme edges of the soles and heels to grip on any small roughness on steeply sloping rock, as in these images: .
Hobnail: Chambers's etymological dictionary of the English language: 
Stud: Chambers's etymological dictionary of the English language: