What you have been given is a bag of dehydrated sourdough starter.
What is sourdough starter? Sourdough starter is a live mixture of yeast and lactobacillus bacteria that will make your bread rise. It will impart that classic sourdough flavor to your bread. It was what was used throughout history to make dough rise.
How do you use it?
Rehydrate your starter. Mix your starter with 2 oz. of water and let it sit on the counter for about 2-3 hours until it is fully dissolved.
Feed it (mix it with) with 1 oz of unbleached flour and cover it. Let it sit undisturbed in a warm spot on your counter overnight. You should see a couple bubbles form after 24 hours.
Add another 1 oz of water and 1 oz of flour. Watch for bubbles.
After 8 hours, feed it another 1 oz of water and 1 oz of flour. You should see it increase by 50% over the next 12 hours. If so, it may be ready to use.
Follow the instructions for Feeding and Maintaining Sourdough Starter.