A tap cuts a thread on the inside surface of a hole, creating a female surface which functions like a nut.
There are three different types.
Bottoming Tap: has a continuous cutting edge with no taper. This allows a bottoming tap to cut threads to the bottom of a blind hole.
Plug Tap: has tapered cutting edges, which assist in aligning and starting the tap into an untapped hole. .
Taper Tap: has a more pronounced taper to the cutting edges.
You can buy them in a set or single at most auto parts stores. Price depends on quality.