Static routes are manually configured network routes.  

A typical use for a static route is a static default route for systems that do not make use of DHCP or dynamic routing protocols: 

set protocols static route 0.0.0.0/0 next-hop 203.0.113.1 distance '1'

Another common use of static routes is to blackhole (drop) traffic.  In the example below, RFC 1918 private IP networks are set as blackhole routes.  This does not prevent networks within these segments from being used, since the most specific route is always used.  It does, however, prevent traffic to unknown private networks from leaving the router.  Commonly refereed to as leaking. 

set protocols static route 10.0.0.0/8 blackhole distance '254'
set protocols static route 172.16.0.0/12 blackhole distance '254'
set protocols static route 192.168.0.0/16 blackhole distance '254'

Note that routes with a distance of 255 are effectively disabled and not installed into the kernel.