Chia seeds are little, tiny seeds that can usually be found in those pull-down bins that you measure out by weight in many grocery or specialty food stores. They are usually black, but here are some that are lighter. They can be ground into chia seed meal with a coffee grinder.
Mixed with water, chia seed meal “gels” together, and (like flax seed meal) makes a good substitute for eggs in baking recipes. Some people like chia seeds even better than flax seeds, since they are tasteless and can easily blend into different types of recipes. For even more info on chia seeds, check out this webpage.