While many actions can occur only on a creature’s turn, there are two types that can happen at any point in a round: free actions and reactions.
Free actions take so little time or concentration that they do not count for any time during combat. Such examples include talking, to a limited amount, or dropping an object. You have an unlimited number of free actions in a round within reason. For example, while talking is a free action, you cannot deliver a twenty-minute monologue in six seconds.
Reactions occur in response to some external event, such as someone moving or trying something that takes attention. Everyone has one reaction a round, unless they have gained extra from some source, such as the Quick Reflexes trait. Reactions can be spent to perform certain action such as Aid or Attack of Opportunity.