Vigilance is used as a measure of how aware a character is of their surroundings.
Vigilance can be low, average, or high.
Low vigilance means the character is not very aware of their surroundings. There may be something impairing their senses or distracting them. When a character has low vigilance, they are most likely to be ambushed.
Average vigilance means the character is paying some attention to their surroundings. This is typical in most situations.
High vigilance means the character is paying very close attention to their surroundings. They are acutely aware of what's around them and ready to respond to it.