The word "Kudo" comes from Greek and means "praise", which is exactly what Kudos are meant for.
These cards help you to show gratitude and can be given to anyone or a whole team. They can also be anything you want them to be, a fun activity with jokes or more serious. But in the end, they are meant to be a simple way to show appreciation, give feedback and motivate your peers.
Kudo Cards can be given out at any time or they could also be part of a retrospective. They can be given out anonymously or you can add a sender to the card, it's really up to you to decide.