It means signals can only propagate about 6 centimeters during a single CPU clock cycle. That makes accessing information off-chip difficult in a single cycle, since 6 centimeters is on the same order as the linear scale of most motherboards.

This limit doesn't immediately impact computations that are carried out entirely on-chip. I expect that's the great majority of computations on today's CPUs, which typically have large caches. So I'm not sure the GP's explanation is correct, although it's certainly interesting.

Nothing can't travel faster than the speed of light – including Electric current. So in a hypothetic 5 gigahertz CPU, there couldn't be a path through the wires/transistors longer than 5.99584916 – otherwise, the end result couldn't be correctly determined. That's why CPU's got smaller and smaller. But since the transistor shrinking slows down, and Adders/Multiplyer/Register/Decoding Logic etc. all takes some time and length, it's harder to make a CPU with a shorter longest path.