This is an 3' long trailer brake hose line.  The rubber flexible hose is DOT approved for hydraulic trailer brakes.  A common use for this brake hose is on boat trailers, but of course it can be used on any trailer that has hydraulic brakes.  On boat trailers that have brakes on more than one axle, this hose is normally used to jump from the first axle with brakes to the next axle via T-fitings.  This is a high pressure hose rated for drum or disc brakes using surge couplers or electric over hydraulic units.  The hose is UV rated to help to keep it from deteriorating in the sun or salt water.  The ends are brass 3/16" male inverted flare fittings that use 3/8-24 thread which fit most trailer calipers, wheel cylinders, master cylinders and solenoid valves.  The end fittings also swivel which keeps the brake hose from getting twisted while you're tightening up the fittings.