Discuss the importance of countercurrent flow to nephron function.?

If I assume that you aren't doing "high level" biology then I will basically explain it as basic as I can.

In the kidneys what you are referring to is known as the counter current multiplier. In the nephron this occurs in the Loop of Henle. Now you first have the descending loop of Henle and the ascending loop of henle. Now urine coming through the descending limb has very low concentrations of solutes (everything in there except for water). Now between the two loops there is a "solution". This solution between the loops has a LOWER water potential than the water potential inside the descending loop. This allows water to diffuse INTO the solution via osmosis, thus decreasing the water potential inside the descending loop of henle. This now increases the concentration of solutes inside the loop to a max at the bottom of the loop. Now as the ascending loop begins active transport pumps out Na+ Cl- ions outside. Now as we go up and up the ascending limb active transport is going to pump na cl out of the loop into the solution. the concentration of solutes now will decrease in the ascending limb.

so the importance of countercurrent flow is to allow water to be reabsorbed into the body but to allow there to be a gradient for that to happen and to allow the concentration of the urine to become more concentrate with urea yet at the same overall concentration it had coming into the loop of henle. (urea goes in at the descending limb here)