Each round of combat is considered to take 5 seconds.
Initiative
At the start of combat, every participant rolls for initiative (1d20 + MOV + P). This determines the combatants' turn order, with the higher rollers acting first. If another creature joins the combat partway through, they roll initiative as well and can act at times in accordance with their roll.
Surprise Rounds
Sometimes abilities or other circumstances will allow some combatants to get the upper hand on their foes by surprising them at the start of combat. When this happens, everyone still rolls for initiative, but the creatures that benefit from the surprise round are the only ones that get to act. Everyone else has their turns skipped.