SYN flood is an unsophisticated DoS attack in which a high volume of TCP connection requests (with the SYN flag set) are directed at some open port on the target. When target tries to acknowledge each request, no third message in the TCP three-way handshake is forthcoming, so the target needs to maintain the half-open connection until it times out. As more and more SYN requests pile in, eventually inbound buffers on the target fill up and fail.
SYN flood is analogous to calling a phone number repeatedly and never responding to the attempted answer. In its crudest form, SYN flood leaves an obvious forensic trail and can defeated by dropping offending IP source traffic at the network perimeter. Also, common firewall or ACL settings of prohibiting inbound TCP connection requests can reduce exposure to this type of attack.