Can anyone please explain the mechanism …

A counter-current mechanism system is a mechanism that expends energy to create a concentration gradient. ... For example, it can refer to the process that is underlying the process of urine concentration, that is, the production of hyperosmotic urine by the mammalian kidney.  The counter-current mechanism takes place in Juxtamedullary nephron. The function of the counter-current multiplier is to produce the hyperosmotic Medullary Interstitium. The ADH promotes water reabsorption through the walls of the distal convoluted tubule and collecting duct. Mammals have the ability to produce concentrated urine. The Henle's loop and vasa recta plays significant role in concentrating urine. The flow of filtrate in two limb of Henle's loop and blood flow in two limbs of vasa recta are in opposite direction hence form counter current.