Most people pick their health plan in about fifteen minutes every fall, and the savings account attached to it never gets a second look. That's the one account that gets three tax breaks instead of the usual one.
A health savings account takes money in before tax, grows it with no tax bill along the way, and lets it come back out untaxed when it pays a qualified medical expense. Your 401(k) gives you the first two. A Roth gives you the last two. The HSA is the only one that gives you all three.
You qualify only if your health plan meets the IRS definition of a high-deductible plan, and that threshold gets adjusted most years. Your plan documents will say whether it's HSA-eligible, in those words. There's an annual contribution limit, a higher one for family coverage, and a catch-up amount once you turn 55. Anything your employer contributes counts against the same limit.
Here's where one dollar does two jobs. A lot of people run the HSA like a checking account for copays and spend it flat every December. If your cash flow can absorb this year's medical bills out of pocket, leave the HSA invested and file the receipts instead. There's no deadline on reimbursing yourself. A receipt from this year can pull tax-free money out twenty years from now, and the account spent those twenty years growing.
A few limits to know before you load it up. Once you're enrolled in Medicare you can't contribute anymore, though you can still spend the balance. Take money out for something non-medical before 65 and you'll owe income tax plus a 20% penalty. After 65 the penalty goes away and a non-medical withdrawal just gets taxed like a traditional IRA withdrawal. I'm a licensed insurance broker, not a CPA, so run the specifics past your tax preparer before you change your contributions.
Where this sits against your 401(k) match, your debt, and your emergency fund is a separate question, and I laid out the order I'd use in which dollar goes where.