What Is a Void-of-Course Moon?
While the Moon crosses a sign, it makes exact aspects to the Sun and the planets. After the last of them it makes no more until it enters the next sign, and that stretch is the void of course. The Moon is quick: in November it spends between 48 and 61 hours in each sign, so a void comes round every two to three days.
To take the current sign as an example (as of 16:05 UTC): the Moon's last aspect in ♏ Scorpio is a square to Jupiter at 23:25 UTC on November 9. From that moment until it enters Sagittarius at 08:35 UTC on November 10, a span of 9 hours 11 minutes, it makes no other exact aspect, so it is void.
The times here count conjunctions, sextiles, squares, trines and oppositions to the Sun and the planets through Pluto. Astrologers who count only the planets known before 1781 get earlier start times on some days; the void-of-course calendar shows by how much for the whole year. How positions are calculated is set out on the methodology page.