One important thing to remember is that people on this website, myself included, are going to overthink the term, and care, much more than will anyone at the event you are attending. The message they likely intend to send is: you should dress up a little. Better than jeans and a polo shirt. But you don't need to wear a suit. I interpret "smart" as giving a little more leeway to something you might wear out in the evening to an upscale club or a trendy restaurant. I don't wear denim at work, but going to a restaurant I might wear dark denim with some nice boots, a white OCBD, and a casual cotton sportcoat, just as an example.