More at its core is that it means that the expansion exceeded the cosmic speed limit, the speed of light. Two of the competing theories have been a rapid expans

More at its core is that it means that the expansion exceeded the cosmic speed limit, the speed of light. Two of the competing theories have been a rapid expansion (inflation) and another that two branes collided and the big bang started in an expanded state. As of late, it appears there has been more confirmation of the former rather than the latter. From what I've read, it is as another commenter noted -- the universe in it's earliest state did not yet conform to the rules of physics as we know them today. This early state is something on the order of the first trillionth of a trillionth of a trillionth of a second.