First of all, the title to Emily Dickinson's poem, "Tell all the Truth but tell it slant," draws the reader's attention to the word "Truth," capitalized in the title. So the focus of the poem is on some essence of the truth. However, where Dickinson generally is straightforward in her ideas of appropriate social and moral behavior, in this instance, she demonstrates a deep insight into the nature of human beings. Unpleasantness is not sometime people enjoy. She does not support lying, but simply appeals to the reader to use a kinder approach in delivering the "Truth."
In essence, Dickinson relays our attention to how we tell the truth. We can throw it in someone's face or tell it on the "slant," coming through the back door rather than charging in the front. This makes the truth easier to take.
Imagery used is found in references to light: the Truth must dazzle, or every man be blind. These words are chosen to allow the reader to understand that the Truth is bound in light, something to be desired. However, sometimes the truth can be debilitating when presented too abruptly. The imagery here also compares the dazzling light which enchants us, as opposed the blinding light that hinders or damages us.
A metaphor used is found in referring to the delivery of truth as a planned activity: it is presented in a concrete terms rather than in abstracts. The metaphorical comparison is found in telling the Truth with a slant (in an indirect or circuitous way) in the same way lightening is described to children so they will not be afraid.