The methodology when using taps for cutting a new thread may mean you need up to 3 different taps.
The first - the taper tap will go into the hole yet to be threaded - the 'tapping size' hole drilled for that purpose.
This tap is undersize at the bottom and will, as it cuts, increase the depth of the thread and bring the diameter out to the correct size as well, because of its tapered shape.
Because of the tapered nature of the first tap, you may wish on thicker materials to ensure the tapped hole is parallel in its sides and thread, and to bring the threads out to their finished size follow the taper tap with a 2nd cut which will perform that job.
The 2nd cut has a small point on the bottom, so it will not fully tap a blind hole.
For this you need a plug tap. This version will cut a thread to the bottom of blind hole, but is the most easily broken if you do not pay attention to the depth of that hole.
To put it simply if the tap gets to the bottom of the thread and you keep turning, it has nowhere else to go, so it snaps.
Remember it is good practice if cutting threads in thick materials to turn the tap only half a turn at a time and then back off a quarter of a turn, allowing the swarf to clear.
The definition of thick material is twice the diameter of the tap, and a lubricant helps considerably.