The truth isn't the easiest thing to explain, especially when you're trying to explain just how to tell it, so Dickinson lends us a helping hand by placing a simile about lightning right smack dab in the middle of the poem. The simile runs through the entire second half, actually:
As Lightning to the Children eased With explanation kind The Truth must dazzle gradually Or every man be blind— (5-8)
In other words, kids can be soothed when you gently explain what lightning is and how it works. In the same way, you've got to unveil the truth bit by bit, or it'll be too hot, and too bright, and just plain too much to handle.