ISRO test of Reusable Launch Vehicle-Technology Demonstrator. It is space plane prototype launched on atop of single booster of PSLV. RLV will be launched for about 48 km, separate from S9 and continue flight due the inertia. After reaching 70 km altitude it will start to entry and reach hypersonic speed. On specified altitude it will deploy parachute and fall into Bay of Bengal. Test is necessary for ISRO to develop space plane program; objectives of test flight are aerodynamics of RLV, utilized materials, recovery possibilities. S9 will be modified for this mission by equipping it with stabilizing fins and secondary injection thrust vector control. You can read more about this mission here.