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.