Choose source and destination IP-addresses (or ranges of IP-addresses) here.

If not needed - set to "Any".

(Available only if any IP-based protocol is selected on Protocols tab)

If "Enable reverse checking" option checked, rule can do following:

if rule source IP(s) and packet source IP comparison does not evaluates to True and rule destination IP(s) and packet destination IP comparison does not evaluates to True - rule makes the reversed check with opposite source and destination addesses. It allows to use one rule for both directions of packets.