Excuse me, what is an ACK message?

In the TCP protocol

Before the communication has to go through three times handshake, the requester sends out a SYN signal request connection, the other party receives and accepts when will send an ACK message.
An ACK is the meaning of a response. What is an ACK attack?
Land Attack attack: In land attacks, hackers use a specially crafted SYN packet-its original address and destination address are set to a single server address for attack. This will cause the receiving server to send syn-ack messages to its own address, as a result, this address sends back an ACK message and creates an empty connection, and each such connection is kept until the timeout, and in land attacks many Unix will crash and NT become extremely slow (about five minutes).