Coupled and Coupling Reactions
A simplistic answer is as follows (though, please do refer to the links given by Greg in a comment to the question):

According to the University of Wisconsin page Coupling Reactions, coupling reactions are


  a reactant-favored reaction is linked to a product-favored reaction so that both reactions yield products.


A main point in the related University of Wisconsin website is that if the reaction is reactant-favoured ($\Delta{G} &gt; 0$), then it is not going to happen, so it is coupled with a product-favoured reaction ($\Delta{G} &lt; 0$), leading to coupled reactions are


  When chemical reactions are coupled, the free energy released by the exergonic reaction is used to drive the endergonic one toward the formation of products.


where the overall reation is product-favoured.

(The website uses an analogy of a coupled train).

A great explanation is in this YouTube clip from Dr. John Pollard, of the University of Arizona.