Every major open-source project has its own style guide: a set of conventions (sometimes arbitrary) about how to write code for that project. It is much easier to understand a large codebase when all the code in it is in a consistent style.
“Style” covers a lot of ground, from “use camelCase for variable names” to “never use global variables” to “never use exceptions.” This project (google/styleguide) links to the style guidelines we use for Google code. If you are modifying a project that originated at Google, you may be pointed to this page to see the style guides that apply to that project.
Google Developer Documentation Style Guide