This is how proficient you are with a given skill, armor, or weapon.
You are either “Unskilled”, “Skilled”, “Professional”, or “Exceptional” at something. Depending on how skilled you are at something, you use a different formula:
Unskilled: Add a relevant ability modifier.
Skilled: Add a relevant ability modifier + your level.
Professional: Add a relevant ability modifier + your level + 3.
Exceptional: Add a relevant ability modifier + your level + 6.
Negatives: If you have a negative modifier, it imposes a penalty to your proficiency.