Dickinson doesn't waste any time telling readers what's up. This is the crux of the entire poem. She thinks the truth is super-important, but should be delivered gently, and at an angle.
Too bright for our infirm Delight The Truth's superb surprise (3–4)
Whatever the truth is, it's bright. Frankly, we're too weak to handle it straight up. If the truth is a raw oyster on the half shell (delicious and pure as it may be), we need to douse it in cocktail sauce and a little lemon juice to take it down.
The Truth must dazzle gradually (7)
You wouldn't want to stare at the sun from three feet away. As humans, we need time, distance, and a seriously protective ozone layer to enjoy the sun. The truth's the same way. While its brightness is the fuel of our very existence, we need it in small doses if we're going to be able to handle its power.