Surprise

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) ability checks of anyone Hiding with the passive Wisdom (Perception) score of the creature on the opposing side with the highest perception score. If creatures are on alert and watching closely, they make an active Wisdom (Perception) check contesting the hidden creature's stealth roll.
Creatures who don't notice a threat are surprised at the start of the encounter.
If you're surprised, you can't move or take an action on your first turn of the combat, and you can't take a reaction until that turn ends.