I had such a situation that, due to an incorrect diagnosis, I had to receive monetary compensation. I got a promissory note. What it is?

Look, as I understand it, it's kind of like a promise that you'll get the money by a specified date. It seems to me that this is a good option, how you can protect yourself if the payer suddenly turns out to be dishonest. In general, I can recommend you the site https://promissorynotetemplates.org/new-york/, where you can read about the promissory note, its types, as well as templates, if you suddenly need it in the future.