Sneak

The character becomes invisible to all monsters that fail a Spot and Listen skill check versus your Hide and Move Silently skills. These skills do nothing unless you are in Sneak mode.

Additionally the number of eyes next to your character indicate how visible you are (1 to 5 eyes, 5 being the most visible, you may need to zoom out to see these).

Also, when a nearby monster's Listen skill exceeds your Move Silently, you will see red puffs appearing around your feet ("Loud Footstep" markers). The monster will not immediately see you, but will instead make a Search action, and possibly find you. If you are heard, you can still possibly duck around a corner to evade the Search. Monsters like spiders, slimes, and some undead have Tremorsense, and always hear footsteps. (Sometimes, seeing the loud footstep markers can be your first clue that an enemy is nearby)