Extract value from one dropdownlist to another

Yii 2.0
1

I have two tables location and department and dropdowns for them respectively , I want when i select location from 1st dropdown then it shows the related departments for that location in 2nd dropdown. I saw a code online for AJAX but i don’t know ajax much and having difficulty understanding the code.

Here is my view

<?= $form->field($model, 'deptloc')->dropDownList(
            ArrayHelper::map(\app\models\Location::find()->all(), 'loc', 'loc'), // Flat array ('id'=>'label')
            ['prompt' => 'Select Location',
            'onchange'=>'
                $.get( "'.Url::toRoute('/category/subcats').'", { id: $(this).val() } )
                    .done(function( data ) {
                        $( "#'.Html::getInputId($model, 'deptname').'" ).html( data );
                        }
                        );
                        ','class' => 'form-control'    
            ]
        ) 
        ?>

    <?= $form->field($model, 'deptid')->dropDownList(
        ArrayHelper::map(\app\models\Department::find()->all(), 'deptid', 'deptname'), // Flat array ('id'=>'label')
        ['prompt' => 'select','class' => 'form-control']
    ) 
    ?>

What is Url route in this? what url should i pass there?
Please Help, and also do i have to make changes to other files as well? or only in view.