Install iOS 11.2.1 for Apple Pay Cash
The HomeKit security update is the only known fix on board iOS 11.2.1. That's all you'll get if you're moving from iOS 11.2.
If you're moving from something older, your iOS 11.2.1 update will be more substantial thanks to iOS 11.2. The features, fixes and enhancements from iOS 11.2 will be included in your iOS 11.2.1 update.
If you skipped iOS 11.2, your iOS 11.2.1 update will enable Apple's new Apple Pay Cash solution.
Apple Pay Cash is the company's answer to popular mobile payments solutions like Venmo. As such, the service allows you to quickly and easily send and receive Apple Pay payments to and from your friends within the Messages app.
When you get receive a payment, the money you receive is kept in Apple Pay Cash and it becomes available for you to send to someone or spend with Apple Pay in stores, in apps, or on the web. You can also transfer it to your bank account.
It might not replace your favorite payment app (Venmo, PayPal, etc) right off the bat, but it might win you over as time goes on.
If you want to use it, you have to be running iOS 11.2 or iOS 11.2.1.