Since the macOS update to 10.13.4, Apple have been showing alerts when old or un-updated applications are being launched (img.1).
It is to warn that 32-bit applications will not run on any operating system after High Sierra, it was nearly a year ago that application developers were initially told of this, so there has been plenty of time for developers to address this issue. Since January any new applications or application updates submitted to the Mac App Store have been required to be 64-bit.
You can check which applications are 32-bit on your system by going to About This Mac in the Apple Menu and then clicking on System Report. Once this is done in the left column in Software click on Applications. When the results populate there is a 64-bit column in the top panel, I had to increase the size of the window to make this column visible. There will either be a yes or no here.
If there are applications that are not 64-bit they will not run on the next major macOS update (10.14). You may want to contact the developer to see if the application will be updated, find an alternative application or not upgrade to the next major version of macOS, usually released in the autumn.
It’s worth checking and preparing now.