A hand tap is used to make threads in a drilled hole, usually but not exclusively in metal.
The tap looks like a bolt as it has threads but the thread are tapered from narrow at the point to the standard bolt thread size as you go up and the tap is usually made from a very hardened steel. Which means they are very hard but can snap or break easily. So do not use excessive force. It is better to take smaller cutting turns then described below then to force and break the tap off in the hole. As the tap is hardened it is almost impossible to drill out or remove one is has been forced and broken off in a hole. The tap usually has a cross bar at the top to allow ease of turning the tap. There are tap charts to look up the drill bit size to use for drilling the hole for final tap size.
There are many size taps depending on bolt size. There is also a regular through tap and a bottom tap. When using a bottom tap is it recommended to use a through tap first to get the threads started, then use the bottom tap to finish the job. A bottom tap is used to tap a hole that does not go all the way through the material (thus past the taper on a through tap) but the threads should reach as close to the bottom of the hole as possible.
For steel the most important thing is to keep the tap and hole well lubricated with oil. More and often is better then not enough. A good cutting oil is recommended. Once the hole is drilled insert the tap into the hole and rotate it clockwise (for standard bolts) gently until you feel it bite trying to keep it as perpendicular to the work as possible. Then rotate it clockwise another 1/4 turn. Turn it counterclockwise 1/4 turn. This breaks away the cut metal from the cutting threads. Again rotate it clockwise for 1/2 a turn and then counterclockwise 1/4 turn. Repeat the 1/2 turn clockwise and 1/4 turn counterclockwise until the depth desired is reached or the entire hole has been taped which is easy to tell as there will be no more pressure resistance on the 1/2 turn clockwise cutting motion. The tape can then be backed out.
Some soft materials like aluminum that are thin do not need the counterclockwise backout cuts an can be tapped simply by continuing to turn clockwise. But as thickness and hardness of the material increases this step will be needed. A final note is that if you try to tap without the counterclockwise backup cut for quite a few turns and then find you need to use it as the cutting it getting very hard, it may be impossible to do the backout cut. So it is better to use the backout cut if unsure.