Flushing is removing the nutrients stored in the grow medium and in the plant.

Like if you stop feeding them N the last 2 weeks of flowering before you start to flush the plant will use up it stored chlorophyll and cure faster and the THC content will increase. (just learned that myself)

I use a commercial flusher to remove the nutrients from my plant prior to harvest, Advanced final flush or GH flora Kleen are good choices.