With the announcement of iOS 11 Apple reached something of a milestone, iOS will be 64-bit only when the new version lands this autumn. It's been an open secret for a while now, however with Apple formalising things you'll now need to think about taking some action.
For most users this won't be a particularly challenging problem, they'll carry on working as before.
For some users though this will cause problems if apps they rely on have not been updated to 64-bit. The most likely reason for this would be that the app's developer has abandoned it and no longer provides support or updates.
To check whether any of your apps will be affected is straightforward. Open Settings... General... About... Applications.
Any apps which need updates to work with iOS 11 will be identified here. If the app is still currently supported hopefully the developer is on the case and it will get updated.
If not you have two options, find an alternative app or, of one doesn't exist, put off upgrading to iOS 11 until an alternative becomes available.