Define kidney reabsorption.

In the nephrons, fluid is filtered through the glomerulus and Bowman's capsule.  This filtrate is similar to blood plasma minus the proteins.  It does not resemble urine at all.  If this filtrate were to go straight to the bladder and out of your body, you would be losing far too much fluid from your body.  This is why reabsorption is important.  Kidney reabsorption is the process that moves solutes (like sodium) and water out of the filtrate and back into your bloodstream.  It is called "reabsorption" because it is technically the second time that these items are being absorbed into the bloodstream.  The first time that they were absorbed happened in the digestive tract after being consumed in a meal of some kind.  
https://mcb.berkeley.edu/courses/mcb135e/kidneyprocess.html

https://upload.wikimedia.org/wikipedia/commons/2/2b/Physiology_of_Nephron.png