Reamers are jewelry making tools used to enlarge the size of the hole in a bead. The reamer is used by placing the bit into the bead hole and spinning it around. The abrasive nature of the bit removes some of the bead material causing the hole to become larger. Because the inside of the hole in beads is abrasive and can saw on your bead stringing thread, it is often a good idea to ream out the hole in your beads just to smooth out any abrasive areas inside the bead.
View reamer jewelry tools here.