If you're using an iPhone running on iOS 9 or older, you won't be able to use WhatsApp anymore. A support document on WhatsApp's site states that its users will be to using at least an iOS 10, which means you'll need to have at least an iPhone 5 or later model. If you're using an older device, WhatsApp recommends you export your chat history before losing access to the app.
The change will only affect a small number of iOS users. As iMore points out, only two percent of iPhone users are still running iOS 12 or earlier versions.