Many software projects produce large, complex programs. In order to handle this complexity, programs are often developed as a set of modules. Each module is coded and tested independently. Only when each individual module passes inspection is the program tested as a whole.
Each module should be as independent from one another as possible in order to reduce the chances of an error in one module producing side effects in other modules.
Modules that are very independent are said to be 'loosely coupled'. Modules that are dependent on one another are 'tightly coupled'.
When the modules have all been written and checked, the compiler translates each module's source code into machine code.
These machine code versions of modules are called 'object files'.
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Click on this link: What is modular programming?