The Mercy was a mechanic that was introduced in Mortal Kombat 3 where the winner of the match could show mercy to the loser at the end of the match, which resulted in the loser receiving a small part of their health back which they could use to fight the winner further. The Mercy returns in Mortal Kombat 11, serving the same purpose.
In Mortal Kombat 3 a Mercy could only have only been done if the winner won the third round of the match. After that, the winner would have to go past-half screen, hold the run button, press down 3 times, and then release the run button. After this the mercy sign would appear and the announcer would say 'character shows mercy', and the loser of the match gains a small bit of health back. If the winner proceeded to beat their opponent again, they would get the opportunity to perform an Animality on their opponent. In Mortal Kombat 11 the Mercy behaves in the same way as in Mortal Kombat 3, except that the player has to hold the flip stance button, press down 3 times, and then release the flip stance button from mid range.