A linker is a software tool that takes separate pieces of compiled code, often called object files, and combines them into a single executable program. It ensures that all references between different parts of the code are correctly connected, so that functions and variables used in one part can be found in another. This process is essential for building complex software from smaller, reusable components and libraries. Without a linker, programmers would have to manually connect all code sections, making software development much more difficult and error-prone.