Taxes filed but got Letter 6475 after

As stated, I filed my 2021 taxes but when I checked the mail after I got home I received Letter 6475. Letters like this is new to me and according to what I researched, I can claim more money with this letter. However I already filed my taxes for this year. So what am I supposed to do with this?

 

F.Y.I. I already received my 3rd Stimulus check last year in case anyone asks.