Characters typically make a certain amount of noise, and thus alert opponents of their presence.
But thieves, as well as characters able to move quietly because of a magical device such as boots of elvenkind have a chance to be absolutely silent when moving. This chance to be absolutely silent is given as a percentage, +/- modifiers,
and the character must roll percentile dice to score less than or equal to the percentage chance he or she has to move without sound. Success indicates silent movement and an improved chance to surprise an opponent or slip past it.