When buying jewelry on eBay it has lot 20 what does that mean?

It means that you are purchasing a "lot" consisting of 20 pieces of jewelry. It is most common to find these deals in the "Wholesale Lots" category. Be careful of these as it may seem to be a great deal to get pen knives, toys, electronics...etc this way, but it sometimes increases the shipping cost according to the amount of pieces in the lot.