iptables --tcp-option ! 2

I keep seeing this in firewall scripts on the net, but I am unable to find an explanation or listing/table of
tcp-options.
The command in question is the following

iptables -A INPUT -p tcp --tcp-option ! 2 -j REJECT --reject-with tcp-reset

Why are [we] only allowing tcp-options of 2? what are tcp packets with option 2? what are the other options, and why
do we not want them?

I'm sure it's safe, and likely a good idea to have in, given the number of tutorials that have it in, but I just dislike
the idea of having something in my to be firewall script that I have little understanding of.

thanks in advance!
-rp

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