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

stuck_on_decoders.elm

import Html exposing (..)
import Html.App as App
import Html.Attributes exposing (..)
import Html.Events exposing (..)
import Json.Decode as Json
import String


main =
  App.beginnerProgram
    { model = model
    , update = update
    , view = view
    }


type alias Model =
  { level : Int
  , name : String
  }
  
  
model : Model
model =
  { level = 0
  , name = ""
  }


type Msg
  = ModifyLevel String
  | ModifyName String
  
  
update : Msg -> Model -> Model
update msg model =
  case msg of
    ModifyLevel newLevel ->
      { model
        | level = Result.withDefault model.level <| String.toInt newLevel
      }
      
    ModifyName newName ->
      { model | name = newName }


view : Model -> Html Msg
view 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))
        [0..9]

  in
    div []
      [ p []
        [ strong [] [ text "Name:" ]
        , span [] [ text model.name ]
        ]
      , p []
        [ strong [] [ text "Level:" ]
        , span [] [ text (toString model.level) ]
        ]
      , select
        [ name "character-level"
        , onChange ModifyLevel
        ]
        levelOptions
      ]
      
      
onChange tagger =
  on "change" (Json.map tagger targetValue)