A round is an in-game unit of time that lasts six seconds. In addition to being a general unit of time (such as for the duration of spells), a round is the basic reference time interval for combat.
Combat rounds are defined per creature rather than being global throughout the module (or even throughout a combat). Each creature's individual combat round is started the moment it makes an attack (melee or ranged) or uses an item. (Casting a spell does not start a combat round. It is possible to attack immediately after a spell is cast.)