"Diría" is the conditional tense from "decir". Basically, we use it with unreal situations, like would+say in English. You can also use conditional tenses in order to sound more formal.
-(Él) No diría eso si ella estuviera aquí. He wouldn't say that if she were here.
-Yo diría que puedes hacerlo mejor. I would say that you can do it better. (more formal and less forthright than: "puedes hacerlo mejor")
I hope it helps!