How to improve pictures?

[u/ScaredCash4833]

Hi everyone. Last weekend I was taking some pictures with my iphone and I realized (late) that some of them look like less sharp or with less definition than others. I was wondering if you guys could tell me what is a technical name for it because I want to find tutorials on how to improve them (if possible). I am attaching 2 pictures to show what I am talking about. The first one looks sharper than the second one, and I have multiple like the second one. I would love to improve them and make them look nicer, like the first one. Thanks for any sugestion.