The first rule allows TCP packets from any source address and port greater than 1023 on the Internet to the destination address of 123.4.5.6 and port of 23 at the site. The second and third rules work in a similar fashion, except packets to destination addresses 123.4.5.7 and 123.4.5.8, and port 25 for SMTP, are permitted. The fifth rule permits NTP traffic, which uses UDP as opposed to TCP, from any source to any destination address at the site. Finally, the sixth rule denies all other packets - if this rule weren't present, the router may or may not deny all subsequent packets. This is a very basic example of packet filtering.
Actual rules permit more complex filtering and greater flexibility.