A self-directed IRA is simply a kind of retirement account (similar to traditional IRAs or Roth IRAs) that allows you to invest in a broader selection of asset classes.
You can either open up a self-directed traditional IRA, in which you make tax-deductible contributions, or a self-directed Roth IRA, in which you make tax-free contributions.
For example, in 2022, the maximum contribution limit is $6,000 (or $7,000 if you're 50 years old or older), and you can only withdraw funds once you reach 59 and a half years old.
Typically you can go to a brokerage firm to open up a regular IRA, but many mainstream brokerages don't offer the option of opening a self-directed IRA.