$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);
 

$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');
 $data['products'] = implode(',',$this->getRequest()->getPost('category')); 
        $model->setData($data);
 

$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')); 

$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);