The WooCommerce software is free to download and plug in to a WordPress website. The core WordPress content management system (CMS) software is also free.
These systems are free to download because they are built and distributed as open-source software. Many clever people around the world (including us) contribute to its ongoing development.
Being free to download is obviously one of the reasons the software is so popular, but it is also very good website software, with a proven history and good track record of eCommerce success.