Someone opens a forgotten diary, dusty and yellowed, and finds a single sheet of paper tucked between the pages. It turns out to be a poem written thirteen years earlier. Reading it again, the speaker is struck by how beautiful the words still are, so beautiful that no amount of time could make them stop loving it.
Then the poem turns into a metaphor. The loved person becomes the poem, and the speaker becomes the diary that holds it. The "spilled ink" suggests that this person has left a permanent mark on the speaker's "paper heart," something that can't be wiped away. The last line, "I keep you safe between my pages," is quiet and protective.
The mood is tender and nostalgic, with a little sadness in it. Nothing is dramatic here. It's the feeling of finding something old and realizing it still matters, and that you've been carrying it all along.