The full truth should be told cautiously especially if it is hurtful or shocking and it should be told in increments so that the person is not offended. The poem suggests that truth told in moderation is the best way.
We must be kind with the truth as if we are telling something to a child and uses lightning as an example. We do not tell a child that lightning can kill you, we just say it is dangerous.
Dickinson’s main point seems to be that the teller of the truth must adjust what they say according to their audience and be candid in their approach. A lesson we can all benefit from.
Tell all the Truth
Tell all the Truth but tell it slant—
Success in Circuit lies
Too bright for our infirm Delight
The Truth’s superb surprise
As Lightening to the Children eased
With explanation kind
The Truth must dazzle gradually
Or every man be blind-
Emily Dickinson (1830-1886)