Routers route, that’s what they do. A packet comes in and a packet goes out. What happens to the packet containing your data inside the router?, how does the router decide which output interface to transmit the packet out of?, what if the router has two routes to the same destination how does it deal with that decision making process?. For a router to make a routing decision it needs to know how to get to the destination address which is described in the header of the IP Packet. Without this knowledge the router could ultimately drop the packet. Our first job is to populate the routers routing table with relevant routes, we say relevant since routers will believe and trust any route placed into it’s routing table and use it resulting in routing blackholes where traffic is lost in the network where the time to live in the packet headers expire and the traffic is then dropped.

See the rest here:
Understand the Routing Process, Part 2