WooCommerce is a free add-on that turns an ordinary WordPress website into a full online shop. Once it is in place, the site can show products, take orders, accept payments and handle delivery and stock.
Think of it as fitting out an empty shop floor: WordPress is the building, and WooCommerce brings in the shelves, the price tags, the checkout counter and the till — everything you need to actually sell. You add your products, and customers can browse and buy them online.
Technically WooCommerce is a Plugin — a ready-made add-on — which is why it installs onto an existing WordPress site instead of being a separate program. It is one of the most widely used ways to build an online store in the world.