Every creature has a proficiency bonus determined by its level (for PCs) or its challenge rating (for monsters and most NPCs). The bonus is used for ability checks, saving throws, and attack rolls when a creature has a relevant proficiency. When applicable, you add your proficiency bonus to a d20 roll. If two different rules say you can add your proficiency bonus to a roll, you still only add the bonus once. Some rules might modify your proficiency bonus before it is applied to a roll; for example, a bard’s Jack-of-All-Trades feature halves the proficiency bonus before it is applied to ability checks where the bard wouldn’t usually add the proficiency bonus at all. If multiple rules modify the proficiency bonus in the same way, you still only modify it that way once.