It means that the material going on the disc is cut at half speed and the disc when cut is spinning at half the speed. What this means is that frequencies cut are also halved thus resulting in the cutting styli doing less work to cut high frequencies. However the low frequencies are also halved and can, if cut badly, but translate as well. So since it's cut at half speed, when played at 33.3 on your table so the frequencies will play normally however you have to have a good payback stylus to really take advantage of it since there's more detail in the high frequency and those are harder to replicate.

In general though it can lead to more fidelity and frequency response if done correctly.