Hi,
That simply means that when a router receives a packet, it will scan its routing table for the best route to use for routing that packet. The best route is the one that has the longest prefix. For example, say a router has two routes, 10.1.0.0/16 and 10.1.1.0/24. When it receives a packet with a destination address of 10.1.1.100, it could use either of the 2 routes in order to route it. However, the /24 route is more specific (i.e has a prefix length of 24 compared to 16) and therefore, that is the route that is used.
Paresh
