Precilla's poem is a small confession about love, built on a play with the word "first." The speaker admits that the person they're addressing wasn't their first partner, at least in the usual, chronological sense. But in the second line, "first" takes on a different meaning: this person was the first to make them feel what a real first is supposed to be like.
The speaker seems to be saying that earlier experiences counted on paper but didn't carry the weight the word promises. Only now do they understand what that weight is. The tone is quiet, tender and a little amazed, as if the speaker is still working out the idea while saying it. The short lines and simple wording add to that feeling of honesty, with no decoration getting in the way. The mood is warm and grateful, with a hint of wonder at discovering that a first can be about emotion rather than order.