Adequate lighting and exposure; good color saturation, pleasant/smiling facial expression, lots of stuff.   Too much for a quickie post.   There are tons of books about this.   Look up the texts used for college level Intro to Photography courses for a good starting point.   Amazon will have tons of listings with reviews.   That shot was almost undoubtedly done with a cellphone camera, but for a spur-of-the-moment candid phone shot it’s a better than a lot of what I see.