This is a difficult poem to understand. I will share my opinion of it. I do believe that poetry is a very personal experience. People may perceive the same poem differently and may discover different meanings or messages. That's fine. Do not fear sharing your opinions on a poem; there is no one right answer. You should express your personal view and understanding whatever that may be. The point of these discussions is not so much about the answer but more about the thinking process and the associations that you are making in your mind with the ideas and thoughts found in the poem.
The very first few lines of the poem really give us the first prominent idea in the poem. The first is that paper is significant and a source of hope. "Paper that lets the light shine through, this is what could alter things."
"Paper that lets the light through," as further described in the poem, is about paper that has been used so much or has been around for so long that it has become thin, almost transparent. That's how it allows light through.
The second important idea is that paper, despite being important, is fragile. "If buildings were paper, I might feel their drift, see how easily they fall away on a sigh..." We can't make buildings with paper; paper is thin and light and doesn't last very long.
The third important idea is that we humans have chosen this fragile thing called paper to record our histories. We print everything on paper, records of our existence ("where a hand has written in the names and histories, who was born to whom, the height and weight, who died where and how, on which sepia date"), maps of our geography and structures ("Maps too. The sun shines through their borderlines, the marks that rivers make, roads, railtracks, mountainfolds") and even our transactions and finances ("Fine slips from grocery shops that say how much was sold and what was paid by credit card").
The end of the poem ties all of these ideas together. We can infer different meanings from the last four lines. What I personally understand from it is that human existence or at least the record and memory of human existence is not eternal. It doesn't last very long; it's fragile and temporary.
You may understand something different however and you should try to make your own inferences about what this poem means. Perhaps "paper" in the poem is a metaphor for something else. Perhaps it is quite literal. It's really your understanding, worldview and imagination that will make a meaning of it.