I don't understand custom event handlers and decoders in Elm

stuck_on_decoders.elm

type alias Model =
  { level : Int
  , name : String
  }

type Msg
  = ModifyLevel Int
  | ModifyName String

levelOptionsView : Model -> Html Msg
levelOptionsView model =
  let
    levelDecoder =
      Json.at ["target", "value"] Json.int
    levelOption level isSelected =
      option
        [ value level
        , selected isSelected
        ]
        [ text level ]
    levelOptions =
      List.map
        (\level -> levelOption (toString level) (level == model.level))
        StatTables.levelList
  in
    select
      [ name "character-level"
      , on "change" (Json.map ModifyLevel levelDecoder)
      ]
      levelOptions

Since event.target.value is a string in HTML, you'll have to parse the value as a string and convert it later. See my fork for an example (you can copy/paste it into elm-lang.org/try).