A SYN flood is a type of Level 4 (Transport Layer) network attack (see Kali/Level 4 Attacks for details). The basic idea is to keep a server busy with idle connections, resulting in a maxed-out number of connections and a resulting denial of service.

hping utility: http://www.binarytides.com/tcp-syn-flood-dos-attack-with-hping/

code a syn flood in C: http://www.binarytides.com/syn-flood-dos-attack/

code a syn flood in Python: http://www.binarytides.com/python-syn-flood-program-raw-sockets-linux/

code a syn flood in Perl: http://www.binarytides.com/perl-syn-flood-program-raw-sockets-linux/