The poem begins with the speaker instructing the reader to "Tell all the truth but tell it slant." The word "slant" indicates a specific angle or perspective, so it's as if the speaker is saying, "Tell the truth, but not directly."
The word "slant" often describes a slope, but here essentially means to approach the truth from an angle rather than head-on—to tell it indirectly. People must find their own unique ways of articulating meaningful ideas.
Of course, "slant" can also refer to a person's bias or predisposition toward a certain idea. In the context of the poem, this might suggest that people have their own viewpoints and their own experiences of the truth—and, maybe, that the truth itself is subjective
Finally, the word "slant" foreshadows the poem's central metaphor, which compares the experience of encountering profound truths to the experience of seeing "dazzl[ing]" light. In the same way that people are better off gazing at a "slant" of light than staring directly into the sun, it's necessary to approach the truth in a roundabout, indirect way.
The first line sets up some expectations for the poem's rhythm. The poem is written in common meter, which alternates between lines of iambic tetrameter and iambic trimeter. A line of iambic tetrameter is made up of four iambs, which are metrical feet consisting of an unstressed syllable followed by a stressed syllable (da-DUM). A line of iambic trimeter, on the other hand, contains only three iambs (three da-DUMs).
This first line, then, is written in iambic tetrameter:
Tell all | the truth | but tell | it slant —
Many church hymns use common meter, so this rhythm lends the poem a sense of importance or reverence while also making it feel more musical.
The presence of heavy consonance and alliteration in the repetition of /t/ and /l/ sounds make the line all the more memorable:
Tell all the truth but tell it slant —
Everything that follows the em dash at the end of line 1 explains or illustrates this point made at the outset of the poem. In this way, the first line functions like a thesis that the rest of the poem will support.