What does "ignorance is bliss" mean?

What you don't know won't hurt you. For example, She decided not to read the critics' reviews—ignorance is bliss. Although its truth may be dubious at best, this idea has been expressed since ancient times. The actual wording, however, comes from Thomas Gray's poem, “Ode on a Distant Prospect of Eton College” (1742): “Where ignorance is bliss, 'tis folly to be wise.”