The degree to which the program module relies on each of the other module.
if you want to change a code or to fix a bug in any one module, let it be class, div, we have to go to other parts of the code base to wherever we have used the same one, to change it. Coupling makes software hard to maintain.
React's idea is to reduce this by using COMPONENT based development.
The degree to which the elements of a module belong together.