Send out syncookies when the syn backlog queue of a socket overflows. This is to prevent against the common "syn flood attack". Disabled (0) by default.
Note, that syncookies is fallback facility. It must not be used to help highly loaded servers to stand against legal connection rate. If you see synflood warnings in your logs, but investigation shows that they occur because of overload with legal connections, you should tune another parameters until this warning disappear. See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.
syncookies seriously violate TCP protocol, do not allow to use TCP extensions, can result in serious degradation of some services (for example SMTP relaying), visible not by you, but your clients and relays, contacting you. While you see synflood warnings in logs not being really flooded, your server is seriously misconfigured.