If you enroll in an HSA eligible insurance plan, you can contribute to an HSA. An HSA is a little like an IRA, it is an account that you can contribute money to, that you can then spend on medical expenses. Your contributions are tax deductible like an IRA. You may also be able to invest in mutual funds or other investments (inside the HSA) that have higher yield than a typical savings account. Money you invest, that you do not spend, remains in the account (it is always yours and does not expire) year after year.
If you withdraw money to spend on qualified medical expenses, the withdrawal is tax-free. So when used for medical expenses, it's tax-free going in and tax-free coming out. You can use the account to pay for medical expenses for you and your spouse, even if your spouse is not covered by your HSA-eligible insurance plan.
If you are covered by a single (you only) health insurance plan, your contribution limit for 2019 is $3500, plus $1000 catch-up for being over age 55.
When you are covered by any other insurance plan or medical benefit, you can't contribute to an HSA, even if your plan is HSA eligible. So your spouse has to have single-only coverage. And, when you turn 65 and join Medicare, you can no longer contribute to the HSA. But, you can spend the money on medical expenses any time, even when you are not allowed to contribute any more.
Also, when you turn 65, you are eligible to withdraw money for any reason and pay regular income tax with no additional penalty (like an IRA), and you can still withdraw money for medical expenses totally tax-free. So, when you turn 65, it's like having a super IRA with a tax-free bonus option for medical expenses.
The basic idea is this. Suppose you can choose a full featured health insurance plan that is $6000 for premiums, and a $2000 deductible, or an HSA eligible plan that as $2000 premiums and a $6000 deductible. Then you put $4500 in the HSA account. If you get sick, both plans cost $8000 for coverage (premium plus deductible)--except you get a tax deduction for the $4500 HSA contribution and you probably don't get a deduction for the $2000 out of pocket from the traditional plan because of the limits on medical deductions. But if you don't get sick, the full plan costs you $6000, and the HSA-eligible plan costs $2000 because the other $4500 is in your account for you to use in the future.
So over the next 4 years or so before Medicare kicks in, you could invest about $20,000 in the HSA. If you get sick, you spend it tax-free for medical expenses. If you don't get sick, you keep the money and can spend it when you retire for more medical expenses (tax-free) like prescriptions and part D premiums, or you can spend it on anything else and pay regular tax like an IRA.
