What is the purpose of a SYN flood?

The purpose of a SYN flood is to fill up connection buffers at the operating system level. The three-way handshake is a process used to establish a TCP connection between a client and a server. It involves the exchange of SYN (synchronize) and ACK (acknowledge) messages.
In a SYN flood attack, the attacker sends a large number of SYN messages to the target server, but does not respond to the subsequent ACK messages from the server. This causes the server to keep the half-open connections in its memory and wait for the ACK that never arrives. As a result, the server's connection buffers become filled with these incomplete connections.