Administrative Distance is a measure used by routers to select the best path when there are two or more different routes to the same destination from more than one routing protocol.
The route which is having the lowest AD value will make its entry in the routing table.
N.B.- If a specific route is being learned by a routing protocol which is having higher AD value will be entered in the routing table instead of the summary route being learned by a routing protocol which has lower AD value due to the longest prefix match rule of routing.
The following will provide the default AD value of different routing protocol:
Directly Connected Routes: 0
Static route out an interface: 1
Static route to Next-Hop address: 1
EIGRP Summary Route: 5
External BGP: 20
Internal EIGRP: 90
External EIGRP: 170
Internal BGP: 200
DHCP Learned: 254
N.B.- If the Administrative Distance is 255, the router does not believe the source of that route and does not install the route into the routing table.