Short answer: Actuarial value (AV) is the share of total covered costs a plan pays on average across a standard population. The ACA metal tiers are defined by AV: Bronze ~60%, Silver ~70%, Gold ~80%, Platinum ~90%; the member pays the rest.
Actuarial value measures how generous a plan is: a 70% AV plan is expected to pay about 70% of covered medical costs across a typical group, leaving members to pay about 30% through deductibles, copays, and coinsurance. The ACA uses AV to define metal tiers; Bronze ≈ 60%, Silver ≈ 70%, Gold ≈ 80%, Platinum ≈ 90% (within allowed ranges). A higher metal tier means a higher premium but lower out-of-pocket costs when you use care. AV is an average across a standard population, not a guarantee of what any one person will pay.