It means when there is a gap between the closing SP of the previous day and the open.
ie SP of share at close is 15c and then it opens at 20c after some great news release after market close.
This is a gap up and is dangerous, because no shares were trading bewteen 15c and 20c hence most of the traders who have bought would be in the money, hence there is a great likelihood that the sellers will rule as they take profits.
Obvious it works in reverse for a gap down. In this case, the buyers will dominate the sellers to close the gap.
