What is a "Cross-Platform" Application?
Cross-platform means that an application, game, or other software has more than one supported target platform. For example, an app that supports Windows, macOS, Linux, and BSD, or both Android and iOS.
Web applications and Electron applications are indirectly cross-platform by having the web browser as their target platform. If it runs on the browser, and there is a browser for every operating system, then it runs on every operating system.
Applications that support Linux are normally cross-platform because the two major GUI development tools available for Linux, Qt and GTK, are both cross-platform. On the other hand, applications made for Windows tend to use toolkits that don't officially support Linux, so they aren't normally cross-platform, although it's sometimes possible to run Windows applications on Linux through the use of WINE.