code: protected function renderLabel()
      {
          $label = $this->encodeLabel ? Html::encode($this->label) : $this->label;
          return $this->hasModel()
              ? Html::activeLabel($this->model, $this->attribute, $this->labelOptions)
              : Html::label($label, $this->getId(), $this->labelOptions);
      }
docs: Renders checkbox label
  
  @return string the generated label