An executable is an external program that Bamboo uses during the build process. Generally, executables compile source code to generate compiled executable files (referred to as artifacts in Bamboo). Ant, Maven, MSBuild or PHPUnit are just some examples of executables that can be used as part of your build process.
New executables can be defined as capabilities in Bamboo. Once an executable has been defined in Bamboo, it can be configured as part of a task.