Dickinson was one slick lady. She was not shy at all when it came to writing about the "big" ideas like love, grief, death, and truth. And she was boss enough to be able to write about these big ideas on many different levels. In this poem, Truth could be (and likely is) many things—the truth about the meaning of life, living your life in a true and honest way, the truth of God, etc.
There are many ways to interpret that capital T "Truth." We even suspect that telling the truth "slant" is an extended metaphor for writing poetry. She knows, as a poet, that it's important to "dazzle gradually" for a poem to make a strong impact on its reader. That's just what she does—she unravels, line by line, the brilliance of the poem with measured patience so we're not overwhelmed or confused. In the end we're revealed a sort of epiphany—or, to put in another way: truth.