What happens if you buy more shares when you’re already in the negative?

I bought a few shares of Nvidia six months (barely anything 5% of my portfolio) I think I paid 416 for three shares someone told me to buy three more shares and old level out I’ve only lost $100 out of 400 which is fun money anyways.

New To the stock market I normally deal with CDs & just my 401k I did some dabbling when I was 19

I’m so lost in the sauce. Please tell me how this works.

Also any book recommendations

Thank u for ur time :)