In the opening scene, Tony Stark said this to peter Parker in car.
Don't do anything I would do, and don't do anything I wouldn't do. There's a little gray area in there. That's where you operate.
What exactly does the bolded part mean?
It's the somewhat tired joke of someone saying the standard phrase of "Don't do anything I wouldn't do" (implying that they should be on good behavior) only to realize that that doesn't cover much, generally followed by "Never mind. Don't do anything I would do," except that Tony is self-aware enough that he starts with saying to not follow his example, and then thinks to add to not do things that he wouldn't consider doing (which is assumed to be really bad things rather than being responsible).
It's a joke.
Remember that this is set after Civil War(captain-america-civil-war) in the MCU (indeed, you kinda need to watch it before this one for some of the context). The main premise of that movie was the tension between Stark (who believed that the Avengers should always be accountable to governments) and Captain America (who believed they should not).
Stark is being his flippant-yet-serious self here. Old Tony (circa Iron Man 1 and 2) never cared about the rules. New Tony (post-Civil War) has to care about the rules (because he kinda went to war with his friends over it), but he also had to admit that Captain America was also right in that there's a limit to how much following the rules can go.
You could rewrite what he's saying as
You need to follow the rules. Except when you shouldn't
Stark's way of putting it is just for comedic value.