This is the ingredient that stops a shampoo from leaving your hair a tangled clump. It is made from guar gum, modified to be both positively charged and lightly water-binding.
It works on a principle called deposition. While you are washing, the polymer stays dissolved in the lather. But the moment you begin rinsing and the concentration of cleansers drops, the polymer comes out of solution and settles on the hair. It is deposited precisely where you rinse, and only in the amount the hair can take up.
The result is hair that feels smooth already in the shower rather than squeaking and tangling. It settles preferentially on the worn, negatively charged areas, so it conditions where the need is, without weighing healthy hair down.