Vibram Sole Guards or Sole Protectors are a thin pieces of high wearing rubber that come in a few colors. People put sole protectors on their shoes to keep the original leather sole from wearing out. When the protector wears out, you just replace it, instead of the sole. Another reason people put sole protectors on their shoes is for gripping. Leather can slide easily, and rubber underneath the ball of your foot gives you a secure grip when walking.
More examples on the way as I slowly add to this website.