A linker is a tool used in computer programming that combines various pieces of code and libraries into a single executable program. It takes object files generated by a compiler and links them together, resolving references between different parts of the code. This process ensures that all functions and variables are correctly connected, allowing the program to run smoothly. Linkers can also handle libraries, which are collections of pre-written code that developers can use to save time. By linking these libraries, programmers can incorporate complex functionalities without having to write everything from scratch. This makes software development more efficient and manageable.