Start with the idea
A generic parameter lets a type implement a trait many times; an associated type lets it implement it once with a chosen output. Iterator uses Item this way, which is why you write Iterator<Item = u32> rather than passing the element type at every call site.
An associated type is a type chosen by a particular implementation of a trait. Iterator::Item is the classic example: once a type implements Iterator, its Item is fixed for that implementation. A caller can write a bound such as I: Iterator<Item = u32> to say exactly which stream it accepts. This is different from giving the trait a type parameter that might allow the same implementer to implement several versions.
In the Container example, Bag chooses String for Item. The get method then returns Option<&String>, borrowing an element from the bag rather than copying it.
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