Before the start of combat, the GM determines if one of the groups (characters or enemies) has successfully surprised the other. This can depend on factors such as stealth, vigilance, or tactics employed.
If group A surprises group B, the characters of group B are considered "surprised" at the beginning of combat. This means that they cannot act during the first round of combat, while group A can act normally.
After the first round of combat, the surprise ends, and the characters of group B can act normally in the subsequent rounds.
Characters from group B can still react with defensive actions (such as dodging) during the first round of combat, but they cannot initiate offensive actions or spells.