Transients are a way of storing cached data in the WordPress DB temporarily by giving it a name and a time frame after which it will expire and be deleted. This helps improve WordPress performance and speed up your website while reducing the overall server load. Transients housekeeping is now part of WordPress core, as of version 4.9, so no need to clean up them manually unless you have specific needs.
There are three types of transients:
1) Expired transients: Are transients that are expired and still exist in the database. These ones can be safely cleaned. You can use the plugin Advanced Database Cleaner to clean them.
2) Transients with an expiration: Are transients that are still valid and used by core/plugins/themes. They should not be cleaned unless you know what you are doing!
3) Transients with no expiration: Are transients that are (maybe) still valid but they have not any expiration date. They also should not be cleaned!