The ".o" file extension represents an object file, which is a binary file produced by a compiler during the process of converting source code into machine code. These files contain compiled code that is not yet linked into a complete program. Object files typically include code, data, and information needed for linking, such as symbols and relocation information. Object files are essential in programming, especially in languages like C and C++. They allow developers to compile code in separate modules, which can then be linked together to create an executable file. This modular approach helps in managing large codebases and facilitates code reuse.