x1 is an executable/binary file to be run in system/runtime environment x2.
A calque on the English word "binary". The x1 refers more generally to executables, not necessarily binaries. This includes shell scripts and interpreted languages such as Python. The x2 refers to runtime environments, which can be operating systems or more general environments such as virtual machines like the JVM. See also samrkompli, which is specifically about compiled programs.