EBuild is the new cross-platform build tool chain for Elements 10. Eventually, EBuild (and with it large portions of the Elements-specific tasks and tool chain for the four platforms) will be open-source and available on GitHub.
Ebuild replaces our use of MSBuild and xbuild within Fire, Water and Visual Studio, and allows us to be much more flexible and innovative with our build process. It is already the default build chain for everything except when building UWP and WinRT projects in Visual Studio.
Fire and Water also use EBuild internally not just for the build process itself, but also for resolving references inside the IDE. This is most notable with the advanced display and handling of Gradle and NuGet package references, or the new resolve log view you can see when selecting the References folder.
You can read a lot more about EBuild in its Documentation section.