A TCP SYN attack, also known as a SYN flood, is a form of denial-of-service (DoS) attack where an attacker sends a series of SYN requests to a system in an attempt to consume enough server resources so that the system is unresponsive to other traffic.
TCP SYN attacks disrupt normal traffic by exploiting the way TCP connections are established. These attacks attempt to exhaust the target system's half open TCP queue, which is a limited resource to service new connection requests. The attacker creates a random source address for each packet and a SYN flag is set in each packet to request to open a new connection, The TCP IP stack of the victim responds to the spoofed IP with SYN ACK and waits for a return ACK from the sender which never comes.
Refer to Configuring use case 1 - protection against TCP SYN attacks.