There are technically two answers here. In commercial vinegar that includes ‘the mother’, this usually means unfiltered vinegar, with sediment remaining at the bottom of the bottle. This is full of healthful bacteria. However, a vinegar ‘mother’ also refers to something that is very similar to that of a kombucha SCOBY. While fermenting, the vinegar will form a scum, which will form into a slimy disk that is referred to as a ‘mother’. This live 'mother' can be used to jump start your next batch of vinegar, allowing it to ferment quicker.