These are kefir grains! When you put these “grains”, a live bacteria/yeast mixture in milk, it ferments. This results in a tart drink called kefir, which is loaded with beneficial yeast and bacteria called probiotics.
Probiotics are found naturally in the body and are considered the “good bacteria/yeasts”. Probiotics help maintain a balance of good vs. bad bacteria, and also help replenish the good bacteria after they are killed by a round of antibiotics. They are a great way to help maintain a healthy gut. Even those who are lactose intolerant can often enjoy this, as the bacteria and yeast consume the lactose in milk.
Put these grains in milk and set it on your counter (unrefrigerated) and you will have kefir in a day or two, depending on how warm your kitchen is. Let it sit out long enough and it even gets a small alcohol content, which is commonly referred to as a bonus. Wink!
The kefir grains pictured above is a live culture and is said to give you a wide spectrum of probiotics (40-60 strains). The stuff in the grocery store is good, but it has been processed and is said to have less variety (around 7 strains).
While many drink kefir straight, I use it as the liquid for my morning smoothie.