Subtype in Scala: what is "type X <: Y"?

Can anyone explain the subtype(<:) in the following code? Why it could be used like that? When we use that? Thanks.

trait SwingApi {

    type ValueChanged <: Event

    val ValueChanged: {
       def unapply(x: Event): Option[TextField]
    }

    type ButtonClicked <: Event

    val ButtonClicked: {
        def unapply(x: Event): Option[Button]
    }

    type TextField <: {
        def text: String
        def subscribe(r: Reaction): Unit
        def unsubscribe(r: Reaction): Unit
    }

    type Button <: {
        def subscribe(r: Reaction): Unit
        def unsubscribe(r: Reaction): Unit
    }

}