Please describe how the kidney concentrates urine?

This is through a mechanism called the countercurrent mechanism at the Loop of Henle that looks like a hair pin. It works by creating a concentration gradient between the nephron and the interstitium which allows water and salt to diffuse and travel out. One side of the loop is called the descending limb as the filtrate travels down through it, and the ascending limb where the filtrate ascends up. A tip is that you have to look at the loop backwards to understand how it works moving forwards and another tip is to know that the mechanism is like a feedback loop that is continuous. 

The filtrate travels through the proximal convoluted tubule and enters down into the descending limb. There the water travels out via diffusion into the interstitium. When it enters the ascending tube it has low fluid and high salt, the salt then leaves through the channels in the ascending loop and what enters the distal convoluted tubule is a very concentrated filtrate full of waste and as little water and salt as possible.

So why does the water and salt move in the directions they do? The ascending limb contains channels that allows sodium and chloride out of the nephron and into the insterstitium of the kidney. This makes the salty environment of the interstium. Remember that water follows salt so the high salt environment of the interstitium then pulls the water out of the nephron through the channels in the descending Loop of Henle. When the water leaves the loop and enters the intersitium, the filtrate still within becomes very salty and concentrated. The body, for blood pressure and other reasons, wants to retain salt therefore it will not want the body to be peeing it out therefore the ascending loop with its sodium and chloride channels does more than just establish the concentration gradient but also helps the body maintain it's salt homeostasis.