Dystroglycan is a protein present in both muscle and brain cells.
The job of the Fukutin-Related Protein (FKRP) is to place a particular kind of sugar on the alpha dystroglycan which enable it to bind to the extracellular matrix.  The extracelluar matrix performs critical functions by supporting muscle cell stability and regeneration.  
With LGMD2i, the FKRP is mutated and doesn't give the dystroglycan enough sticky sugar molecules to adhere.  The image below demonstrates the impaired binding.