A WordPress theme is a collection of files that determine the design and functionality of your website. Themes typically include templates for different types of pages, such as your homepage, blog posts, and contact page. They also contain CSS stylesheets, images, and other resources that help define the look and feel of your site.
Themes can be free or paid, and they can be customized in a variety of ways. Some themes offer a simple set of customization options, such as changing the colors or adding a logo. Others allow you to customize every aspect of your site, from the layout and typography to the navigation and widgets.