A health savings account (HSA) allows you to use tax-free money to pay for qualified medical expenses. The money you use to fund the account is not subject to federal income tax or state income tax in most states (California being one exception), and employer contributions are not taxable to the employee. Unlike a flex savings account (FSA), the money contributed to an HSA can be rolled over from year to year. There is no "use it or lose it" feature. In order to contribute to an HSA, you are required to be enrolled in a high deductible health plan (HDHP). You cannot contribute to an HSA if you are on Medicare Part A, but if you had an existing HSA account, you can use the funds to pay for qualified medical expenses while you are on Medicare. Considering Medicare doesn't pay for all medical expenses in retirement, and that this money is tax-free, there is a planning opportunity to over-fund an HSA with the understanding that you are letting the funds grow to be used tax-free in retirement.
For 2018, the max that can be contributed to an HSA is $3,450 for one person or $6,900 for a family.