In WordPress, a theme is a set of template files and style sheets used to define the appearance and presentation of a site's content. This could be loosely called a site design.
They can be added, modified, and managed from the Appearance > Themes menu. A very large number of free or paid themes are available on the internet. The WordPress.org Themes directory contains many of them, but if you don't find what you're looking for there, you can find more in WordPress Theme Shops.
Each theme has its own design, layout, and functionality. Some are designed to serve a specific type of site, such as photographer themes for example.
A user must choose the one that suits their tastes and needs for their site.
You will find various posts studying themes in detail in the section Themes of WPMarmite.