Roth IRAs phase out at higher incomes. Earn enough, and you're not allowed to contribute directly, full stop. The backdoor Roth is the standard, legal workaround: contribute to a traditional IRA (which has no income limit on the contribution itself, just on deducting it), then convert that same money to a Roth shortly after.
Why it works
There's no income limit on converting an existing traditional IRA to a Roth. The income limit only applies to contributing directly. Route the money through a traditional IRA first, convert it, and you've arrived at a Roth account despite the income limit that would have blocked you from getting there directly.
The part that actually matters
This only stays simple if you don't already have other pre-tax traditional IRA money sitting around. If you do, the conversion gets taxed on a pro-rata basis across all your traditional IRA balances combined, not just the new contribution, which can turn a transaction that was supposed to be tax-free into one with a real tax bill attached. This is the single most common way a backdoor Roth goes wrong, and it's entirely predictable in advance if someone checks first.
What to actually do
Check your existing IRA balances before you do the conversion. If there's old pre-tax money in the picture, there are still options. It just isn't the simple, no-tax version people expect going in.
If you're planning to do this for the first time, or have done it before without checking this, it's worth a conversation before the contribution goes in.
This post is general information, not specific tax, accounting, or legal advice. Your situation may differ, so talk to us before acting on anything here.