An object file is a binary file produced by a compiler after source code is compiled. It contains machine code, which is the low-level code that a computer's processor can execute. Object files typically have extensions like .o or .obj and may also include metadata, such as symbol tables and relocation information, which help link the object file with other files during the linking process. Object files are essential in the software development process, as they allow for modular programming. Developers can compile individual source files into object files, which can then be linked together to create a complete executable program. This modular approach simplifies debugging and enhances code reuse.