It's not a stupid question, it's actually a great question :D
To sum it up, New Adult Fiction is about coming-of-age. Its during the time when teenagers are becoming adults. They're growing up, taking on more responsibility, dating not just to find love but to settle down, some are getting jobs while others are going onto college/trade school. Basically, it's a focus on adult themes (drugs and alcohol abuse, mental health, suicide, family struggles, bullying, marriage, getting a job, entering the military, etc.).
This is why Romance (Colleen Hoover) has taken over the New Adult Fiction by a storm, but NA encompasses ALL genres. For Fantasy, your MC could have graduated wizard school and is now moving on into university wizard school (Magicians). For Science Fiction, your MC could be taking on leadership roles, working as an apprentice, joining the military, or a princess who has now finally grown up is about be wed to some alien race.
The possibilities are endless! Usually books that fall under New Adult Fiction have an MC who is 18-30 years old.
How is this different from Young Adult? It's a step up. You can write about mature themes and have sexual details in your book, when usually, YA has restrictions on how much adult details you can write.