https://www.juniper.net/techpubs/en_US/junos12.1/topics/concept/routing-protocols-route-preferences-overview.html#id-10195528
For unicast routes, the Junos OS routing protocol process uses the information in its routing table, along with the properties set in the configuration file, to choose an active route for each destination. While the Junos OS might know of many routes to a destination, the active route is the preferred route to that destination and is the one that is installed in the forwarding table and used when actually routing packets

http://www.juniper.net/techpubs/software/junos-security/junos-security10.3/junos-security-swconfig-interfaces-and-routing/topic-46938.html

A static route destination address can have multiple next hops associated with it. In this case, multiple routes are inserted into the routing table, and route selection must occur. Because the primary criterion for route selection is the route preference, you can control the routes that are used as the primary route for a particular destination by setting the route preference associated with a particular next hop. The routes with a higher preference are always used to route traffic