Very simply, WordPress is software for creating and managing websites. It is developed and provided by WordPress.org, and it is free thanks to the support of the non-profit WordPress Foundation and the efforts of volunteers who keep the software up-to-date, develop and test new features, write documentation, etc.
Because WordPress is an ongoing project with regular updates, it helps you keep up with changing technology. For example, WordPress makes is easy to use a responsive design that adapts to various screen sizes, from large desktop monitors to small smartphones.
Before you try to use WordPress — or any other tool for creating a website — it’s a good idea to understand some things about how websites work. For example, if you create a website without understanding what a web hosting service is, you could end up having no ownership rights and loosing control of that website. So please get a basic understanding of how hosting services work before you try to use WordPress.