Coupling refers to communication and interchange of information between models.
For example a typical standalone atmospheric model may be initialized with information about sea surface temperature. Depending on the model the ocean may be treated as a heat reservoir and may or may not affect any change to the SST. The ocean itself is not modeled. Similarly an ocean model may parameterize the atmosphere and only care about surface temperature and moisture fluxes (and perhaps momentum fluxes for waves) which may be static or prescribed values.
Contrast this to a coupled model where the atmosphere and ocean models exchange data with each other so that they can evolve as a whole. Rather than the atmosphere seeing a static SST, it sees the modeled SST from the ocean model. Because the ocean is modeled even more information than just SST is available to the atmosphere model.
CESM in particular is a family of models that all talk to each other to model earth, air and water as a whole system.