KPA is a form of cryptanalysis where the attacker can compare plaintext massages with the resulting ciphertext in order to find statistical weaknesses in the cipher or determine the private key. KPA resistance is a considered a must-have property for any modern cipher.
As a fun historical note, the most famous cryptanalysis (or code-breaking) of all time - British cracking of the German Enigma machine during World War 2 was an example of a known plaintext attack. British intelligence used known bits of plaintext - called "cribs" - such as the weather report at the beginning of a message, or the "Heil Hitler" at the end in order to reduce the search-space for breaking each day's private encryption key.
