`$postData['category']` this will be your database table column name and `$this->getRequest()->getPost('category')` your form field name.
implode function will be add `,` between the values.


    $postData['category'] = implode(',',$this->getRequest()->getPost('category')); 


add in controller.php in **saveAction**


     $model = Mage::getModel('brand/brand');
     $data['category'] = implode(',',$this->getRequest()->getPost('category')); 
			$model->setData($data);