What does it mean when a guy says something about you and walks away smiling?

It depends on what he says. If it's something complementing, he's normally still thinking about you, and how wonderful you are; or if you're a guy, he's thinking "did he buy it?" and snickering about it. If he's said something nasty or inconsiderate, he's either a rude, nasty, horrible person, he's trying to get back at you for something, or he's in a bad mood.