Each round represents 15 seconds in the game world. A round presents an opportunity for each character involved in a combat situation to take actions.
Each round’s activity begins with the character with the highest initiative result and then proceeds, in order, from there. Each round of a combat uses the same initiative order. When a character’s turn comes up in the initiative sequence, that character performs one standard action. Characters with more than one standard action continue taking turns within the combat round in the order of initiative until all combatants have no more standard actions remaining. The round ends when all combatants have used their standard actions and the next 15-second combat round begins.
For almost all purposes, there is no relevance to the end of a round or the beginning of a round. A round can be a segment of game time starting with the first character to act and ending with the last, but it usually means a span of time from one round to the same initiative count in the next round. Effects that last a certain number of rounds end just before the same initiative count that they began on.