Need help understanding following commands

 root@bt:/# iptables -t nat --flush root@bt:/# iptables --zero root@bt:/# iptables -A FORWARD --in-interface eth0 -j ACCEPT root@bt:/# iptables -t nat --append POSTROUTING --out-interface eth0 -j MASQUERADE #Forward to a local site root@bt:/# iptables -t nat -A PREROUTING -p tcp --dport 80 --jump DNAT --to-destination <Proxy’s IP>