I believe IP chains cruises down the "chain" of rules until one matches.
then does what ever the target is. It can be another set of rules or
one of the special actions like REJECT. By putting these lines first the
packet matches the rule, and rejects the packet and stops. Order is
important since each rule is examined in the order they appear and if
matched no other action will be taken.
From the ipchains man page
TARGETS
A firewall rule specifies criteria for a packet, and a
target. If the packet does not match, the next rule in
the chain is the examined; if it does match, then the next
rule is specified by the value of the target, which can be
the name of a user-defined chain, or one of the special
values ACCEPT, DENY, REJECT, MASQ, REDIRECT, or RETURN.
In this case the 0/0 notation must mean all hosts. I am used to seeing
it as 0.0.0.0/0 but this is more concise. It threw me a little because
I have not messed with this stuff for a while and had to figure out the
host src and dest notation.
Hope this helps.