How it is calculated
If one drive has a chance p of failing in the period, the chance that none of n drives fails is (1 − p)n, so the chance that at least one fails is 1 − (1 − p)n. The chance of two or more comes from the binomial distribution. Over several years, the yearly rate compounds: p = 1 − (1 − AFR)years.
This assumes drives fail independently. In real arrays they often do not: drives from the same batch, with the same hours, in the same box, tend to fail close together. Treat the result as a lower bound.
What it means for your array
With one drive at 1.5% per year, a failure in any given year is unlikely. With eight drives, the chance that one of them fails in a year is about 11%, and over three years about 30%. That is why every array that holds data you care about needs redundancy, and why RAID 0 gets riskier with every drive.
Real failure rates differ a lot between models and with age. The failure rates by model page lists Backblaze's latest figures for 31 drive models.
Redundancy covers the drive failure. It does not cover deleted files, ransomware or a fire, so keep a backup as well.