WordPress is a website creation tool written in a programming language called PHP. It is, by far, the most popular website platform in the world and roughly one quarter of all websites in the world run on WordPress. If you're buying a new website or if you've bought a website in the last 10 years then, chances are, it is built on WordPress.
WordPress's most valuable feature is its content management system. It simplifies the task of building a website and for this reason most website companies choose to build on WordPress. It also gives the customer access to their website so that they might make changes to the text on their website in future.
WordPress does come with drawbacks but in general it is a good solution for the majority of websites.