Surprise

If a creature is surprised, it can't move or take an action on its first turn of combat, and it can't take a reaction until that turn ends.

The GM determines who is surprised based on Stealth and Perception checks.

A band of adventurers sneaks up on a bandit camp, springing from the trees to attack them. A gelatinous cube glides down a dungeon passage, unnoticed by the adventurers until the cube engulfs one of them. In these situations, one side of the battle gains surprise over the other.

The GM determines who might be surprised. If neither side tries to be stealthy, they automatically notice each other. Otherwise, the GM compares the Dexterity (Stealth) checks of anyone hiding with the passive Wisdom (Perception) score of each creature on the opposing side. Any character or monster that doesn't notice a threat is surprised at the start of the encounter.

A member of a group can be surprised even if the other members aren't.