They have some remastered content that is 'supposed' to sound better with compatible devices/receivers. Could be as basic as cranking up the gain in the recording itself to make it seem louder.
Its a bit like THX certification. Not the end of the world if you don't have it.
You buy an IMAX enhanced receiver, the next thing you'll need is a enhanced TV. Don't waste time and money on this. If you want the IMAX experience, a cinema is the only way you can get it.