Vermiculite is a brownish silicate mineral commonly used to help create air spaces in soil, which improves percolation and nutrient movement. It can also be used on top of freshly planted seeds to retain moisture, which improves germination.
[Note, until 1990, many sources of vermiculite contained asbestos. Today, pure vermiculite does not contain asbestos - whew!]
I hope this information inspires you to grow more of your own food. You can ask your garden questions on my Home page.