Lines 1-4 introduce the argument that Alexander Pope will develop throughout this 18-line stanza, which is a famous (and fairly self-contained) passage from a longer poem called "An Essay on Criticism." Essentially, the passage argues that becoming an expert in the arts is hard and humbling—but necessary in order to have anything worthwhile to say as a critic.
Pope makes this argument via two extended metaphors: one spanning lines 1-4 and the other spanning the rest of the passage. The first metaphor alludes to "the Pierian spring," which, in Greek mythology, was the sacred spring of the nine Muses (goddesses of art and science). Think of it as a fountain of wisdom, or a metaphorical source of artistic knowledge and inspiration.
According to Pope, a critic (or artist) should "Drink deep" from this spring rather than settling for a quick taste:
A little learning is a dangerous thing;
Drink deep, or taste not the Pierian spring:
There shallow draughts intoxicate the brain,
And drinking largely sobers us again.
In other words, "we" (as critics) have to consume a lot of knowledge in order to really know anything at all. Settling for "shallow" knowledge, or "A little learning," is "dangerous" in the sense that it causes us to fool ourselves (and potentially others). It goes to our heads, "intoxicat[ing]" our "brain[s]" with the delusion that we understand more than we actually do. Only by "drinking largely"—studying the subject deeply—do we become "sober," or humble and realistic about the limits of our understanding.
Notice how this metaphor plays ironically on the connotations of "intoxicate" and "sober," words normally associated with alcohol. Unlike alcohol, Pope implies, knowledge makes us more sober as we consume more of it. And through sharply drawn antithesis, Pope makes clear that consuming a lot of knowledge—not "A little"—is exactly what critics should do. In fact, he hints that "Drink[ing] deep" in this way will make critics themselves "deep" rather than "shallow."
Like all of "An Essay on Criticism"—and virtually all of Pope's poetry!—these lines take the form of heroic couplets. In other words, they're rhymed couplets of iambic pentameter (lines that follow a five-beat, "da-DUM, da-DUM" rhythm). These first two couplets function almost as a quatrain, tracing a single complex metaphor across four memorable lines.