The following is taken from the help file which explains "sliding".
Stop Loss
Set the tick offset at the desired number of ticks. If you back at 3.55 with a tick offset of 1 then the stop loss will be set at 3.6, if the best back price reaches 3.6 then a lay bet will be placed at odds of 3.65 (1 tick higher to ensure matched, if it doesn't get matched then change the odds manually). If you tick 'Sliding' then if the market goes your way the stop loss moves down also (ie. if the best back prices goes down to 3.5 then the stop loss goes down to 3.55)