Relations Between Union and States
(1) If the Rajya Sabha passes a resolution (two-thirds majority of those present and voting) declaring it necessary in the national interest, Parliament may legislate on a specified State List matter (or GST) for all or part of India while that resolution is in force. (2) Such a resolution lasts up to one year, renewable for further one-year periods. (3) A law passed under this power ceases to have effect six months after the resolution lapses (without affecting things already done under it).
This lets Parliament temporarily legislate on a normally State-only subject, but only if the Rajya Sabha (which represents the states) specifically authorises it as being in the "national interest" by a two-thirds majority - giving states, through their chamber, a check on how and when this exceptional central intervention into their domain can happen.
This provision gives Parliament a way to address a State List subject that has become a genuine matter of urgent national concern (like a coordinated response to a specific crisis), without needing to first amend the Constitution's basic Union-State division of powers.