Hi Bashaece
DR( Designated Router) is elected when ever OSPF routers are connected to the same multi-access networks. It is done to reduce the number of adjancencies formed. If there is a change in topology the initial router will only updates the DR and BDR (Backup Designated Router). The DR in turn will update the remaining routers.
If the DR is down, then the BDR will act as DR.
If U have any issue, reply back