Overhead code is the additional (or excess) object code generated by a compiler to provide machine code which will be executed by a specific CPU. This code includes translations of generic instructions listed in cross-platform code, and is tailored for execution on a specific platform or architecture. An example of overhead code would be code generated to handle reference counting, while source code written in a high level language is an example cross-platform code.
Analogy
The coverage area of a standalone house is more than the space taken by enclosures. Similarly, overhead code is the part of the program which is not listed in the source code. However, it is needed for the software to operate properly.
References
Overhead code Wikipedia(Text) CC BY-SA