Im new to linux and this is my first post here and i looking for answers for some of my questions
Here is the defaults of my iptables:

target prot opt source destination
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED
ACCEPT icmp -- 0.0.0.0/0 0.0.0.0/0
ACCEPT all -- 0.0.0.0/0 0.0.0.0/0
ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:22
REJECT all -- 0.0.0.0/0 0.0.0.0/0 reject-with icmp-host-prohibited

Chain FORWARD (policy ACCEPT)
target prot opt source destination
REJECT all -- 0.0.0.0/0 0.0.0.0/0 reject-with icmp-host-prohibited

Chain OUTPUT (policy ACCEPT)
target prot opt source destination

i know how iptables work in general but what i cant understand is:
1- why are the source and destination 0.0.0.0/0?

2- what does the following mean:
2.1- reject-with icmp-host-prohibited
2.2- state RELATED,ESTABLISHED
2.3-state NEW tcp dpt:22

3- what is icmp? i know it is an iternet protocol but what is the use of it/when do i use it, Which applications of services use it?.

i tried googling this but i got lost
Thanks in advance and i will really appreciate detailed answers

1 - it means "everything". literally it represents a subnet on 0.0.0.0 with a 0 bit subnet mask, which will match everything.

2.1 - drop the connection and send an ICMP Host Prohibited message back
2.2 - allow connections which are already established or related to established connections
2.3 - allow new connections on TCP port 22 (that's SSH)

3 - it's Internet Control Message Protocol, it's what protocol Pings are, and are used to send low level technical messages.

SO SO much more on wikipedia and the likes, just have a search.