Basically (very basically), while there are various permutations, its an arbitrage strategy that involves selling volatility on the broad market while buying volatility on specific low vol, higher beta names. There are tons of Phd types trying to raise funds with purported proprietary dispersion models. However, I've yet to hear of any of them actually making money with them. Still, I use a quasi-dispersion strategy in my own trading.
Regards,
HD