Not sure about net filteration rate (please recheck the term)
I am going to assume you have the basic knowledge of a nephron's structure.
to summarize, blood flow through a kidney
Renal artery --> multiple divisions --> arcuate artery --> afferent arteriole --> GLOMERULUS (capillary bed #1) --> Efferent arteriole --> peritubular capillaries (capillary bed #2) --> ultimately rejoin to form veins --> which join to form the renal vein --> which drain the kidney.
So you can see that like any other organ, blood enters kidney, and that same blood after passing through the "Artery->arteriole->capillary->venule->Vein" pathway, leaves the kidney. only difference is, a fraction of that blood actually never makes it out of the kidney and is excreted from the body, and in simpler terms we call it the urine
In order to make that urine nephron does a lot of filteration and reabsorption (again i hope you are acquainted with the histology of a nephron).
GFR is all about Glomerulus
as the afferent arteriole breaks into the glomerular capillary bed, it forces the blood out of the capillaries into the bowman's space. this filtered blood is called the "ultrafilterate". The amount of blood filtered into the bowman's capsule per minute is called GFR (glomerular filteration rate).
This is roughly 120 ml/min i.e. 120 ml of blood is filtered through the glomerulus into the bowman's space, every 60secs, adding upto ~180L per day.
Of course you don't pee 180 L per day. So most of this 'ultrafilterate' is reabsorbed, by the peritubular capillaries (the capillary bed #2). as the blood is reabsorbed, so are most of its constituents (eg glucose, Na, Cl, K, etc). but the excretory substances are not reabsorbed (duh!) and they pass out of the body into urine. This is the concept of clearance
Clearance of "X" = (urinary concentration of "x" . urine flow rate) / Plasma conc. of "X"
Clearance is the amount of blood that the kidney frees up of a particular substance ("X" in above formula) in unit time (eg, per minute)
using the above explanation of GFR, assume that the kidney filters 120 ml of blood per minute. and that this blood has "z" concentration of a substance, say urea (just for example).
If the kidney handles urea such that the blood entering kidney has z-urea, and the blood leaving kidney has z-urea (i.e. no change in concentration) then we can say that none of the blood volume was cleared of urea, i.e clearance = 0
If blood leaving kidney has all of its urea removed, i.e. urea conc is zero. that we can say that the kidney removed all of the urea, and since kidney is filtering 120ml of blood per min (GFR), we can therefore say that 120ml of blood is being cleared of urea in one min. clearance = 120 ml/min
Similarly, if blood exiting kidney has half the urea conc (1/2 z), then we can say that only half the filtered urea was excreted, rest was reabsorbed. or in other words, of the volume of blood filtered (GFR=120 ml) half was freed of urea clearance = 60ml/min
PS. this got much lengthier than i thought it would, i hope its worth the read though.