Flushing is when you pour water through your pot so you can rinse the soil. I use a veg nute and a flowering nute.
Before I switch from veg to flower I flush to rinse out the veg nutes. My last watering during flowering is a flush too
as I rinse the flower nutes out of the bud. Second flush is particularly important as the nutes will leave a chemical
taste in your bud if you don't flush.
I generally finish my girls in 3-gallon pots, so, I flush with 5-6 gallons of regular room-temperature water. Cold water
can shock plants. You will have to pour a bit at a time as it takes a couple minutes for 5-6 gallons of water to drain through
a 3-gallon pot. Let her drip drain for 15 minutes before putting her back in circulation or you'll have pan runover, eh?
BigSteve.