Meaning: He is good person to the core, or He may look normal, but he’s no good
There is nothing worse than getting ready to scramble some eggs for breakfast (or Breakfast for Dinner night!), then cracking one open to find something has gone horribly wrong. That’s the basis for this saying. Even though on the outside everything seems normal, a “bad egg” will reveal its inner rottenness soon enough. A "good egg" is the best of what you had hoped for.