As the scene opens, Quince, Starveling, and Flute wonder what has become of Bottom and lament that there is no one else in Athens who could play the part of Pryamus but he.
Here, Bottom is telling Quince and Flute of his odd experiences "I am to discourse wonders" means that he has wonders to relate. He will not tell the assembled group, however, because he is saving the story for the Duke. He would not be a "true Athenian" if he gave such juicy information to the likes of his friends! :)