CPython is the reference implementation of Python that is written in the C programming language. This is the most commonly installed version of Python since it is distributed with many operating systems.
This version of Python is designed with an emphasis on correct and reliable execution of code. It is not the fastest (in terms of performance) implementation of Python but is the most feature complete due to the large amount of development effort that goes into this project.
Since CPython is an open source project you can browse the code over on the CPython GitHub repository.