Cisco Express Forwarding (CEF) provides the ability to switch packets through a device in a very quick and efficient way while also keeping the load on the router's processor low. CEF is made up of two different main components: the Forwarding Information Base (FIB) and the Adjacency Table. These are automatically updated at the same time as the routing table.
The Forwarding Information Base (FIB) contains destination reachability information as well as next hop information. This information is then used by the router to make forwarding decisions.
The FIB allows for very efficient and easy lookups. Below is an example of the FIB table:

The adjacency table is tasked with maintaining the layer 2 next-hop information for the FIB. An example of the adjacency table is shown below:

It uses a fast cache that is maintained in a router data plane' fast cache is only used when fast switching is enabled while CEF is disabled.

The command "ip nat inside source list 1 interface gigabitethernet0/0 overload"
translates all source addresses that pass access list 1, which means 172.16.1.0/24
subnet, into an address assigned to gigabitethernet0/0 interface. Overload keyword
allows to map multiple IP addresses to a single registered IP address (many-toone)
by using different ports so it is called Port Address Translation (PAT).