Proficiency. Proficiency is an important concept. Characters have a proficiency bonus determined by class level. A character's class and background determines a character's various proficiencies.
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If you are proficient with a weapon used in an attack, then you add your proficiency bonus to the attack roll (but not to damage).
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If you use armor or a shield that you lack proficiency with, you have disadvantage on any ability check, saving throw, or attack roll that involves Strength or Dexterity, and you can’t cast spells.
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If you are proficient in a skill or tool that applies to an ability check, then you add your proficiency bonus to the check result.
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If you are proficient in a type of saving throw, then you add your proficiency bonus to your saving throw result.
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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.
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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.