We have no way of knowing why the IRS sent you a check. The IRS will send you a letter explaining what the check is for. You will probably get the letter in two or three weeks.
Here are two common reasons that the IRS would send you a check, but they don't necessarily apply to your particular case. You won't know for sure until you get the IRS letter. If the check is for a small amount, it might be interest on a tax refund that you received earlier. The IRS pays interest if your refund was delayed. If it's a larger amount, it might be a refund for the unemployment exclusion in the American Rescue Plan Act. This would only be the case if you reported unemployment income on your tax return and you filed before TurboTax was updated for the unemployment exclusion.