Aptitude is a simple way of assessing your character’s general level of training and aptitude for a given task. It is broken into three different aptitude levels: untrained, capable, and proficient. Each level grants a different Aptitude Bonus.
Capable. + half your Aptitude Bonus (min 1)
Proficient. + your Aptitude Bonus
Characters have a Aptitude Bonus determined by their level and cannot become proficient until they are 5th level or higher. Creatures also have a Aptitude Bonus, which is incorporated in their stat blocks. The bonus is used in the rules on skill checks and saving throws.
Your Aptitude Bonus can’t be added to a single die roll or other number more than once. For example, if two different rules say you can add your Aptitude Bonus to a Will saving throw, you nevertheless add the bonus only once when you make the save.
Your Aptitude Bonus might be divided (halved, for example) before you apply it. If a circumstance suggests that your Aptitude Bonus applies more than once to the same roll, you still add it only once and divide it only once.