The short answer
It counts how many of 100 people like you would have a heart attack or stroke in the next 10 years.
It comes from a calculator that uses your age, sex, blood pressure, cholesterol, diabetes and smoking. In the US, clinicians often call under 5% low, 5 to 7.5% borderline, 7.5 to 20% intermediate and 20% or more high.
Things the calculator misses, like a family history of early heart disease, high Lp(a) or a calcium score, can move your estimate.
Source: 2018 AHA/ACC cholesterol guideline; AHA PREVENT equations, Circulation 2024.