Token is one of the 3 most popular modules in Drupal.

It allows you to use small placeholders to automatically complete tasks. To take a simple example, if you put [site:name] on your site, it will be replaced by the actual name of your site. To take a more complicated example, you can use Token together with the Pathauto module to automatically create URL patterns for your whole site.

However, Token needs other modules to work. If you want to use Token inside fields, there are several options but one of the most reliable is a module called Token Filter.