Zones of supply and demand are regions where there exists high probability trading opportunities.
Each zone consists of an entry price and a stop price. In the most simple form, retests occur whenever price leaves that zone and then comes back to retest that level. However, for purposes of having a rule based system, there are specific criteria in place to determine when price is considered to have left the zone.
Only when price has first left the zone is it possible for it to be retested. Until that time, the zone is drawn but considered invalid.