It is basically a condition where the dog doesn't have the ability or muscle strength to propel the food into the stomach after swallowing, and if not sitting in the correct position the food can bypass the 'food pipe' and go down the 'wind pipe' into the lung and cause the pup/dog to die.
Here is a link to help understand a bit more and goes into more detail.
http://www.vetstreet.com/care/megaesophagus
Here is a link to help understand a bit more and goes into more detail.
http://www.vetstreet.com/care/megaesophagus