What does it mean to "see the Dhamma"

In a number of the books I'm reading on Buddhism, and in a number of talks I've heard on the subject, I repeatedly encounter the idea that the Buddha "saw the Dhamma" or that he "witnessed Dhamma" or the like. Other times, I've heard/read that the Buddha "saw" beings dying and taking rebirth, or that he "saw" the reality of anicca, dukkha, anatta, and the Four Noble Truths, etc. I'm having difficulty understanding what this all means. Is this metaphorical sight? Is it literal sight, in the sense of a vision or dream? In terms of our own meditation practices, when we are told that we should "see" certain things, e.g. seeing a nimitta, should we expect something similar? Metaphorical "sight"? Literal sight? Something else entirely? Thanks!