Have you ever seen a slimy, gummy, jelly-like substance or layer of film that can form on the top of or in the liquid of your vinegar? This is called the "Mother" of vinegar.
“Mother” of vinegar will naturally occur in vinegar products as the result of the vinegar bacteria itself. Mother is actually cellulose (a natural carbohydrate which is the fiber in foods like celery and lettuce.) It is actually sold and used to start more batches of vinegar! It is similar to the scoby for making batches of kombucha.
While not exactly appealing in appearance, it is completely harmless and the vinegar does not have to be thrown away because of it. Mother of vinegar can be easily filtered out using coffee filter, or, perhaps even better, the mother can simply be left in the vinegar and ignored.