Combat in Lucerne happens over the course of 6 second turns. Each character and non player character, and any other relevant combat entity, is added to the Initiative. See: Turn Order and Initiative.
Combat has a standard set of phases:
When combat begins, so does the Initiative phase. All players and non players roll for their Initiative, and end up in a turn order based on the results. See: Turn Order and Initiative. Once the turn order has been settled, combat begins.
A round of combat is approximately 6 seconds long from the character's point of view. In this time, every character in Initiative may act when on their turn in the turn order. Once the character at the end of the turn order has finished their turn, the round ends, and a new one begins from the top. Some mechanics or abilities trigger at the start or ending of a round of combat to assist with timing.
A turn is, specifically, one characters room for acting within a round. There are many abilities, spells, etc, that trigger at the start or end of a turn in combat.
On your turn, you may perform Actions, Move, and interact with the scenario in any way you and your GM see possible within the confines of the rules, and what they will allow.
At the end of combat, the GM will announce combat has ended, and Initiative / Turn orders end. You are free to describe actions, and the game returns to a more fluid time scale.