Unit is a subtype of scala.AnyVal. There is only one value of type Unit, (), and it is not represented by any object in the underlying runtime system. A method with return type Unit is analogous to a Java method which is declared void.

unit

Syntax:
    Expr1 ::=  ‘return’ [Expr]

A return expression return e must occur inside the body of some enclosing named method or function. The innermost enclosing named method or function in a source program, f, must have an explicitly declared result type, and the type of e must conform to it. The return expression evaluates the expression e and returns its value as the result of f. The evaluation of any statements or expressions following the return expression is omitted. The type of a return expression is scala.Nothing.
The expression e may be omitted. The return expression return is type-checked and evaluated as if it was return().
Read more, section 6.20.

return type return statement

It's a bad practice to declare abstract vars in abstract classes or traits. Do not do this:

trait Foo {
  var value: String
}

Instead, prefer to declare abstract things as def:

trait Foo {
  def value: String
}

// can then be overridden as anything
class Bar(val value: String) extends Foo

The reason has to do with the imposed restriction - a var can only be overridden with a var. The way to allow freedom to choose on inheritance is to use def for abstract members. And why would you impose the restriction to use a var on those that inherit from your interface. def is generic so use it instead.
Read more.

override best practices traits interfaces trait inheritance