The short answer:
In the app, a Kin is a place to share experiences and expenses with other people.
The slightly more detailed answer:
A Kin usually has two or more people (though there’s nothing stopping you from having a Kin just for yourself).
It can have a name that identifies what it’s for - for example, “Trip to Tokyo” or “Our Little Secret” (which we won’t ask you about, because it’s your little secret).
And it can have a purpose that describes what it’s for - for example, “Flights, accommodation, sushi & stuff”.
In a Kin, you can list things you plan to spend money on, and things you’ve already spent money on.
You can record who’s going to pay (or has already paid) for what, how you’re going to split it all, and who owes how much to whom.
You can settle with others in a Kin, and ask them to settle with you.
All in the nicest, possible way.
You can use a Kin for things like…