The alertness skill gives a chance of avoiding ambush. It’s not the only method of doing so, but it’s one of the more reliable.
You can toggle this skill on and off by using it. You may want to do so if you’re a party leader and lack rescue, to keep from running past monsters which can ambush your party members.
The ingame help reads:
Type of skill : neutral skill. Affecting stats: In. It uses no endurance points. One of the biggest advantages one can get over another in a battle, is the advantage of surprise. Many times, surprise allows one to control the battle. Also, being surprised makes it easy for one's foe to keep him from leaving the battle. If one has keen senses and is aware of his surroundings, he can sometimes avoid ambushes, and walk away unscathed. Simply using the skill will cause you to turn the skill on/off.