Many users call this by three different names, Odin counter, Flash counter, or the Download Counter ( Most Common). This counter appears when the device is in download mode. When you use Odin, or anything else to flash a non Samsung Kernel to the device, this counter will change from "NO" to a number, depending on how many times you flash a kernel using this method will depend on what your counter says.
Why should I care about this?
Normally you wouldn't have to care, however If you have to return your device to Samsung or the carrier. Their technicians are now trained to check this download counter and if it says ANYTHING but "NO" your warranty will be voided, they will either return the device unfixed to you or send you an invoice for the repairs.
My device is fine, is there any advantage to clearing the download counter?
If you have a custom recovery flashed, and are confident you will not be using download mode to flash a non Samsung kernel, I say YES. In the event your device becomes damaged and you are unable to flash it. You will have the confidence of sending it back knowing that the Counter reports "NO" so there is less of a chance that the device will be rejected for repair under warranty.
Is this dangerous?
In a perfect world things never go wrong, but our world is not perfect clearly. You are flashing a boot-loader, specifically the SBL1 (secondary boot-loader #1). You should not ever have an issue flashing this. But In the event your battery dies while flashing this or the zip file becomes corrupt. YOU WILL BRICK. This is not designed to let you reset your counter 100 times, each time you reset the counter, you play with fire. Too many times, and you get burnt. CHECK THE MD5sum! Don't be a fool.
How do i check my counter?
Power your device off, hold Vol Down and turn the device on, keep holding the Vol button until you see download mode appear. Press Vol Up to bypass the warning and your "Custom binary" counter will be the second line, it should say "NO" if you are clean and warranty intact, it will contain a number if not.
Ahhhhhhhh... I get it, so what's this flashable zip do again?
It resets your counter from a numbered value to "NO"
Get the Flash Counter Reset CWM Flashable Zip
Fear not, they have been tested.
md5: 7230923eedcdb8268d6d951089eebc7c flash_counter_reset_Telus_SGH-T989D.zi
md5: 3e117d2c5b290fb3f5d6f4ca01ac2691 flash_counter_reset_Tmo_SGH-T989.zip