A SYN Flood sends a large number of SYN requests (the first step in creating a new TCP connection). After this the attacker ignores the ACK which is sent back from the server and simply sends another SYN. The goal is to overload the server with huge number of open TCP connections. By doing so the server will not be able to respond to valid traffic from normal users - thus resulting in a Denial of Service (DOS).
A SYN flood is a form of denial-of-service attack in which an attacker sends a succession of SYN requests to a target's system in an attempt to consume enough server resources to make the system unresponsive to legitimate traffic