A trait is like an interface. A struct implementing a trait means it takes on the methods of that interface (but there's more to it, because a trait can have default implementations of of the trait methods).

If you want to express something like "this is a variable that holds something that fulfils this trait", without knowing the _actual_ type it is, that variable effectively has an unknown runtime size. std::io::Read is an interface for reading bytes of some source, like a file or a socket.

This matters because we're talking about a stack frame. So the size needs to be known at compile time.

    let a: u64 = 42; // ok, because well known size.

    let b: Read = ...; // illegal, because unknown size.

A "trait object" places the object on the heap and has a pointer in its place.

    let b: Box<Read> = ... // legal, because pointer is a known size

However it's a bit more complicated, because this syntax allows for dynamic dispatch at runtime using a vtable. So there's a quite big difference between Box<u64> (a 64 bit unsigned integer on the heap) vs Box<Read> (a runtime dispatched lookup via a vtable).

This difference is not obvious at a glance though. Hence the new syntax: Box<dyn Read>.

(I think I got that right)