CMS means Content Management System, i.e. a system where you can manage all the content of your website.
Content in combination with products is all the necessary product information, such as: images, texts, specifications, etc.
Open source means that the source code of a project is publicly available – and often free of charge. This makes it easy for programmers to develop further as they see fit, on a basic package.
WordPress and WooCommerce are both open source. However, the fact that a programmer claims to be able to develop on such a system does not always mean that this is done neatly. Pay attention to this when hiring a potential programmer.
A plugin is an extra functionality that you can add to your WooCommerce webshop. For example, with a plugin you can add a PDF invoice to your orders or link a third party service such as dropshipping via VidaXL. By adding a plugin, the source code of WordPress and WooCommerce will not be touched, so you can continue to perform updates – generally safely.