"Gapping" refers to an occurrence whereby the market moves from one quoted price and that this second quote is significantly different to the first. Where such an event happens and where the second quote is through an order level (Stop Loss, Limit or New Order), when the first quote was not, this may trigger a Gapping event.
There are a variety of reasons why this might happen. Some of the more usual, because the particular Underlying Market on which the order is placed has opened and started trading at a price significantly different from the previous session closing price; or during trading hours the Underlying Market may have become unusually volatile or illiquid for a period of time causing sudden dramatic price movements; or the Underlying Market may have gapped from one traded price to another, significantly different, traded price due to a piece of economic, political, environmental or corporate news.
Moreover, we recommend clients limit their exposure when holding positions over the weekend or prior to market closing or during release of any political events and be aware that such gaps may occur.