So lost

So I have an awesome internship this summer at one of the largest beef packing companies in the country at one of there plants. I know at HQ the dress code is "business casual"...but at the plant they said "jeans and a casual shirt". WTF is a "casual shirt"? Obviously not like a graphic tee or something like that but I have no idea what to even look for that isn't too dressy or casual to make it still look professional without being stupid cause the plants can be dirty. I am working in Operations but I am still unsure of my specific duties. I am so lost at what direction to go help please(pictures would be fantastic to help know a little bit better what to look for)