Yes, it is just ordinary store bought baking soda - that same stuff that you cook with...sodium bicarbonate.
It is to prevent bloat. Should you leave it out free choice? Well...
Personally my own opinion is to not leave it out free choice. Why? Because it clumps easily. When it clumps up and becomes hard the goats won't touch it when they need it, so you're constantly replacing it. What a waste. I don't like waste. My choice is to administer it when it is needed and not to hope that the goats will self administer.
I do see the school of thought about offering it free choice though. If I were going to leave it out free choice I would do so at select times when bloat is at a higher risk. Such as when new Spring grass is appearing (or after a drought). When changing a goat's feed. When causing a goat stress, such as moving it or during breeding or kidding.