I think this means sometimes static sometimes dynamic. but can you explain some use cases for this?

I am strongly in favor of static, coming from C.

A perfect static type checker would distinguish between valid and invalid programs in all cases. In practice, static type checkers are conservative and occasionally reject valid programs. Gradual typing gives you a way out in these situations -- just like how macros in C can provide type-generic functionality.

A sufficiently rich type system (with tuples, union types, traits, generics, higher-kinded types, etc) can lessen the need for a dynamic escape hatch. In the end though, the occasional use of dynamic typing can be easier to reason about.

Everything is static by default unless you explicitly opt to declare a variable as dynamic for specific use cases.

Also worth noting that Dart now also includes fully sound null safety https://dart.dev/null-safety

If you want to convert your data to JSON you put it into a Map<String, Dynamic>.