That is offering you a small patch that lets iTunes in Mojave continue to be your method of upgrading, synching, and backing up your iOS device without upgrading to Catalina.
See: If you see "Software update is required to connect to your iOS device" on your iPhone, iPad, or iPod touch - Apple Support
It is a quick download that does not change your macOS version at all. I've performed it on two Macs, one with Mojave and the other with High Sierra, and neither OS changed. I have no issues with the computers after the update.
Click the "download" button that is offered.