From Traderpedia

Definition: A form of bet unique to spread betting whereby a trader can preset a stop order to limit potential losses that is guaranteed to be honoured by the spread betting firm whatever happens in the underlying market.

This insurance is rarely worth the premium (usually paid by way of an increased bid/offer spread), but at times when, for instance, the market gaps up or down by a huge margin on astounding news, a trader will be thankful for it.