A double tap
is the act of getting off two successive shots from one sight picture
as quickly as one can pull the trigger.
The idea is to put two bullets on your target as fast as possible to put the target down quickly. This is mostly practiced on pistols because they generally deliver less energy than rifle rounds but learning to double tap a rifle can also be rewarding and useful.
A person who wishes to perform a double tap will aim for center of mass or the chest then fire two shots in quick succession, if done perfectly the first shot will hit the chest and the second the head or thereabouts which hopefully will neutralize the threat and end the encounter. This is because the gun is just coming back to alignment from the recoil of the first round fired so the 2nd shot is expected to hit somewhere above the first one.
In the real world this does not happen very often, more likely your first shot will hit the A or B zone (on an IDPA target) and the 2nd one will hopefully hit close by or at least the C or D zone, this is on a good day at the range real world is a whole different ballgame altogether. Learning to perform double taps can be tricky but fun. The value of this can be immense if you ever need to put somebody down in a hurry.
People have been known to walk away from center of mass hits from even a .357 Magnum JHP. Local LE personnel here have even told me they sometimes feel that their standard issue DAO Beretta 92F in 9mmx19 is quite inadequate and they tell stories of shooting at a perp who just ran away from them even as they got hit center of mass.
I personally know someone who actually walked away from eleven (yes 11!) hits from a .45 caliber pistol at point blank range, center of mass. This person unfortunately later died due to blood loss and haemmorhage.
Sorry we're all out of political correctness, but we do have a lot more of shmolitical correctness. Parley!