First Time Head of Household Filer Needs Help Understanding IRS Transcript Codes and Tax Topic 152 Refund Status

I just got my transcript and honestly I have no idea what I'm looking at. There's all these numbers and codes and dates. Can someone explain what all this means? Is there an easy way to know when I'll get my refund? I checked my refund status on the IRS2Go app and this is what it says: Refund Status Return Received Refund Approved Refund Sent We have received your tax return and it is being processed. If you filed a complete and accurate tax return, your refund should be issued within 21 days of the received date. However, processing may take longer under certain circumstances. Please check here or visit the Refunds page on IRS.gov, to check on your refund status. Please read the following information related to your tax situation: Tax Topic 152, Refund Information Filing Status: Head of Household I'm confused about what the Tax Topic 152 means. Does that give any indication of when I might get my refund? It looks like they've received my return but nothing else has been approved or sent yet. Does the 21-day timeframe start from when they received it? And what are the "certain circumstances" they mention that could make it take longer? I filed as Head of Household if that makes any difference in timing.