The basic idea is that potential consumers for a new product (e.g., smartphones) fall on a spectrum, approximated by a bell curve. Innovators are people who will buy anything that's new and possibly useful, and they'll buy Version 1.0, even though it's riddled with bugs. They're willing to fight bugs for the pleasure of being first. Early Adopters wait until most of the bugs are worked out and buy Version 2.0. The Early Majority waits until the market is proven and there are viable competitors. The Late Majority buys the new product when their old product wears out and can't be replaced. (These are the people who "can't go wrong buying IBM".) The Laggards buy the new product , if ever, about the time that the new product is being obsoleted by an even newer product.
The "crossing the chasm" idea is that there is a huge difference ("the chasm") between early adopters and the rest of the potential purchasers. Tech products often find innovators and early adopters, even without a serious sales effort, but it's much more difficult to convince the remaining potential buyers to become real buyers.