Flushing the soil with large amounts of water to rid the growing medium of as many nutrients as possible. Do a search with "flushing" and you'll get the lowdown.
Ignore him bro. One thing i should have asked, you DWC or soil? If you're soil my previous statement stands lol.
GC has a wonderful thing called a search button. Click it, type in flushing, and you will be amazed at what happens next. Very basic questions like yours come pre-answered here at GC. Seek and ye shall find...