"Decoupling" means that two things that are normally linked are being separated.
For example: A locomotive is decoupled from a waggon. The link is broken.
In the discipline of economics, economic growth is the prevailing paradigm. Most people understand and/or notice that economic growth requires more resources (wich includes energy) and often has undesirable effects on the environment.
"Decoupling" is the idea that economic growth or development could happen without negative effects on the environment.