Genesis and Kadence are "theme frameworks" or "parent themes" which designs leverage when creating a theme.
You will want to use a Genesis theme if you value stability, simplicity, and focusing on creating content.
You will want to use a Kadence theme if you want more control over styling and layout, and have a private developer and don't need theme support. For more information, see our Kadence documentation.
The Feast Plugin comes with access to both our Genesis and Kadence themes so that you can change between them as your needs evolve. Note that the setups are different and will require following different setup instructions if changing.