Surprise Round occurs when a person or group has been able to surprise another group or party. This gives one side a massive advantage.
A surprise round occurs when one side has not been made aware of the other side when they attack. Usually this means one side Stealth check was able to beat the opposing passive/active perception and the hidden side decides to attack the unaware side.
The surprise round is a round that occurs before the rolling of intiative and the attackers can decide how to spend each of their turns in conjunction with each other.
Unless stated otherwise, the surprised group cannot take reactions and do not take a turn.
The attacking group gains advantage on all attacks, and the defender is considered off-guard and have disadvantage on reflex saving throws until the surprise round ends.
Once the attacker has taken all their turns, the surprise round ends and intiative is rolled normally.