Two short lines, and a whole journey sits inside them. The title tells us the speaker is a pilgrim, and the first line says what the journey is for: not a shrine or a faraway city, but "inner being." The speaker is setting out to find something within, which is a strange kind of destination, since you can't reach it by simply walking somewhere.
The second line is the surprise. Instead of promising to write letters home, the speaker says "rain will be my letters." That suggests a traveler who has little use for ordinary messages and who expects the weather to speak in their place. Rain falls, soaks the ground, and leaves no signature, so it may hint at letting go of names, ties and the need to explain oneself. It could also be a quiet way of saying, "I'll be in touch, but not in words."
The mood is calm, spare and a little wistful. The speaker seems willing to be alone, even to disappear, and the poem's brevity echoes that lightness.