well, theres flushing when you wanna get rid of too many chemicals in your soil mix that could cause nutrient burn.
You would simply just water the plant with about double the amount of WATER to rinse out the extra fertilizer. then let it recover.
The other flushing is when you would just give plain WATER to your plants when its about 14 days from harvest. Reason you would do this is to make the plant use up all of its remaining nutrients so when you smoke the buds you don't get a "chemical" or "grassy" taste in the smoke.
Hope it helps!
Cheers