Hey kiddo!
Do you know what a law department is? It's a group of people who help a city or town make sure they follow the rules and laws.
And New York City has its own law department! Just like how you follow rules at home or school, New York City has rules they have to follow too.
The law department helps the city government follow those rules and deals with any legal problems that come up. They have lawyers who give advice to city officials and represent the city in court if they need to.
So basically, the law department makes sure New York City is doing what it's supposed to do and helps it when it needs legal help. Pretty helpful, right?