What is "Element" type?

Here is the definition in the Apple documentation

Element defines a placeholder name for a type to be provided later. This future type can be referred to as Element anywhere within the structure’s definition.

Element is usually used as the generic type name for collections, as in

public struct Array<Element> { ... }

so it is what you construct your array from, and not something predefined by the language.

If we tried trace the idea of how we do even get Element when working with collections, we would notice that it is related to the Iterator protocol. Let's make it more clear:

Swift Collection types (Array, Dictionary and Set) are all conforms to Collection protocol. Therefore, when it comes to the Collection protocol, we can see that the root of it is the Sequence protocol:

A type that provides sequential, iterated access to its elements.

Sequence has an Element and Iterator associated types, declared as:

associatedtype Element

associatedtype Iterator : IteratorProtocol where Iterator.Element == Element

_{You could review it on the Sequence source code.}

As shown, Iterator also has an Element associated type, which is compared with the sequence Element, well what does that means?

IteratorProtocol is the one which does the actual work:

The IteratorProtocol protocol is tightly linked with the Sequence protocol. Sequences provide access to their elements by creating an iterator, which keeps track of its iteration process and returns one element at a time as it advances through the sequence.

So, Element would be the type of returned element to the sequence.

---

Coding:

To make it simple to be understandable, you could implement such a code for simulating the case:

protocol MyProtocol {
    associatedtype MyElement
}

extension MyProtocol where MyElement == String {
    func sayHello() {
        print("Hello")
    }
}

struct MyStruct: MyProtocol {
    typealias MyElement = String
}

MyStruct().sayHello()

Note that -as shown above- implementing an extension to MyProtocol makes MyElement associated type to be sensible for the where-clause.

Therefore sayHello() method would be only available for MyProtocol types (MyStruct in our case) that assign String to MyElement, means that if MyStruct has been implemented as:

struct MyStruct: MyProtocol {
    typealias MyElement = Int
}

you would be not able to:

MyStruct().sayHello()

You should see a compile-time error:

'MyStruct.MyElement' (aka 'Int') is not convertible to 'String'

The same logic when it comes to Swift collection types:

extension Array where Element == String {
    func sayHello() {
        print("Hello")
    }
}