A reamer is a type of finishing tool. When we drill holes in lumps of metal, we typically use a fairly standard drill bit in either our mini-lathe or micro-mill. Drill bits do a pretty good job. If you only need a hole to pass a bolt through, or to tap with a thread, then a standard drill bit will create a suitable hole. A reamer will take a rough hole cut by a drill bit and turn it into a truely round hole of a known inner diameter.