" Middle level output of a compiler "
High level languages like (C,C++,Java,C#) are easy to code , Programmer write code using a text editor or IDE provide by related language compiler called source code.
Source code cannot run direct by computer it need to be transform into machine language by compiler.
Compiler took many steps to transform high level language to machine language. Byte code is middle level output of a compiler. It cannot be read by the human being and cannot be run but after some more steps it change into low level and became ready to run.