How to get system.xml config values?

I am trying to get the value of system.xml from a plugin I am developing.

 <field id="Moduleposition" translate="Module" type="multiselect" sortOrder="30" showInDefault="1" showInWebsite="1" showInStore="1">
                    <label>Seleccionar tienda</label>
                    <source_model>MageplazaSimpleshippingModelConfigSourceConfigOption</source_model>
</field>

From the following function I am initially trying to return a test value to show it inside an alert

 public function getSelectStore()
    {
        return "probando";
    }

I am trying to display the return of this variable within a .phtml file using Jquery.

 alert('<?php echo $block->getSelectStore(); ?>');