Surprise
A side is so caught off guard that the GM may grant the party advantage on Initiative, or in extreme cases forgo rolling Initiative and let each hero start with all 3 actions. Merely being hidden or striking first is not enough—an enemy who is on guard or at all aware of you cannot be surprised. If the heroes are surprised, they may have to roll Initiative with disadvantage, or in extreme cases start combat with only 1 action, or the monsters may all go first.
From Core Rules