NTP is Network time protocol and it’s used to synchronize a computer's time with a time server. NTP uses UDP protocol on port 123 for communication.
In an NTP (reflection) attack, the attacker sends a crafted packet requesting a large amount of data to the time server. NTP commands like Monlist, available on older versions of NTP are used in the crafted packet to get a historical list of computers who connected to the time server. The list of servers can be used thereafter to attack or infect them at a later time.