Rabt literally means connection or linkage. It is the unwritten rule that a sheʿr's two misras must together complete one thought. The first misra sets up — establishes a premise, a scene, a question. The second misra resolves — draws the conclusion, twists the knife, supplies the answer.
Without rabt, a sheʿr is just two rhyming lines next to each other. With rabt, it becomes a tiny machine of meaning. Take Ghalib's famous matla: the first line says thousands of desires, each one big enough to die for; the second says many of my longings were realized, yet they were still too few. Together they produce a single, devastating observation about human appetite. Neither line alone has it.
Rabt is the deepest test of a sheʿr. Great poets — Mir, Ghalib, Faiz, Iqbal — write sheʿrs where you cannot remove one misra without destroying the other. That fusion is the thing.