A WordPress theme changes the design of your website, often including its layout. Changing your theme changes how your site looks on the front-end, i.e. what a visitor sees when they browse to your site on the web. There are thousands of free WordPress themes in the WordPress.org Theme Directory, though many WordPress sites use custom themes.
At YSJ, WordPress websites come with a number of pre-installed Themes available for any sites to use.
LinkedIn Learning: Themes: What are they and how they work