What happened is

Start from Apr 5 2017, with iOS 10.3 or later iOS version, you might see this warning

This app will not work with future version of iOS. 
The developer of this app needs to update it to improve its compatibility

Apple has also required developers submit new apps with 64-bit support since February 2015 and app updates since June 2015.

The switch to 64-bit only support means that older iOS devices built on 32-bit architecture will not be able to upgrade to the new iOS. This includes the iPhone 5, 5c, and older, the standard version of the iPad (so not the Air or the Pro), and the first iPad mini.

What does this warning mean is

In the iOS 11 is your 32-bit app will no longer work on iOS11. You need to build app on arm64 to make it running.
June 28, 2017 64-bit app in iOS 11

What you can do is

Refer to the docs to check your device model and compatibility
iOS Device Support Matrix
iPod Model
iPhone Model
iPad Model

Read the guide for converting to 64-bit Binary Convert your app to 64-bit Binary

How to change your build settings in XCode

In project build settings change architectures from $(ARCHS_STANDARD) to arm64
in your valid architectures settings(both project level and target level), you also need to add arm64 if this is missing there.