If you are proficient in a type of saving throw, then you add your proficiency bonus to your saving throw result.
Your proficiency bonus can’t be added more than once. For example, if two different rules say you can add your proficiency bonus to a Wisdom saving throw, you nevertheless add the bonus only once.
Occasionally, your proficiency bonus might be multiplied or divided (doubled or halved, for example) before you apply it. See, for example, the rogue’s Expertise feature.