Where can I find help for iptables?

Question

linux

Nowhere can I find a detailed description of iptables on the shelves.
There is a jffs script for the router that opens port 9091 for Transmission:

#!/bin/sh
iptables -I INPUT -p tcp --destination-port 9091 -j ACCEPT
iptables -I INPUT -p tcp --destination-port 51413 -j ACCEPT
iptables -I INPUT -p udp --destination-port 51413 -j ACCEPT

What does the -I option do ?
Where can I find a guide to iptables where you do not need to re-read everything in order to understand part of the code, but everything is point by point and with examples?
H/W: ASUS RT-N66U Rev. B1
F/W: AsusWRT-Merlin v.376.48_1
UPD: Why are you so angry? After all, there are times when the head does not cook at all. This resource was created in order to ask and answer, and not to reproach those who ask for curvature.

Answer 1

man iptables

-I, --insert chain [rulenum] rule-specification
    Insert one or more rules in the selected chain as the given rule number. 
    So, if the rule number is 1, the rule or rules are inserted at the  head
    of the chain.  This is also the default if no rule number is specified.

Answer 2

Nuuuu listen, to ask what is in the first lines of the issuance of any search engine - well, it's a shame!

Answer 3

This is just a pipets, how can they ban you like that in Google, you can’t find the answer to this question on the very first link of any search engine, you need to have an ass instead of a head and growing hands from there.