Hi,
As has been pointed out, you can use pool salt, which you can find at Lowe's or other hardware outlets. You can also use solar salt. The Ag designation is something I haven't heard before, and I wouldn't worry about it. It generally is recommended that you not use salt that has YPS (yellow prussiate of soda), which is used as an anti-caking agent in fine salt.

Enjoy your new pool!

Mark