People are usually considered to be aware of their surroundings, especially in combat situations. Sometimes, however, a situation arises where a character remains unaware of enemies lurking nearby (whether out of sight or hidden; see Ambush on page 237). Surprise means a hero is startled by an ambush or the sudden appearance of enemies. Surprised characters cannot defend against the first action taken against them. Once this surprise action (most likely an attack) is resolved, regular combat rounds begin.
Publication: Core Rules page 36