The poem starts by describing the beginnings of the "enterprise" of the poem's title, a complex project with bold intentions. The speaker is part of a group that decides to undertake a pilgrimage. The poem never clarifies the pilgrimage's purpose or destination, which is part of what opens the poem up to so many symbolic/allegorical interpretations. Generally speaking, though, a pilgrimage is a long journey to a holy site that facilitates personal, communal, and/or spiritual growth. A pilgrim, it follows, strives for spiritual transformation through travel.
And when this pilgrimage begins, that's exactly what the speaker and the others seem to want. Their minds become "exalt[ed]"—that is, raised to a happier and nobler state—and problems seem metaphorically "light" (insignificant) in the context of the greater goal.
Yet, even here, there's a hint of irony at play: it shouldn't be the decision to undertake a pilgrimage that really provides spiritual nourishment, but the journey itself. Perhaps like life, the journey is as important, or even more so, than the destination. In other words, the pilgrims are excited by the idea of pilgrimage but haven't yet come face to face with its reality.
The full-stop caesura after "light" signals that this initial optimism may be short-lived, giving the poem a jerky rhythm that goes against the momentum required by a pilgrimage.