Answer:
While the rose exists as a beautiful natural object that has become infected by a worm, it also exists as a literary rose, the conventional symbol of love. The image of the worm resonates with the Biblical serpent and also suggests a phallus. Worms are quintessentially earthbound, and symbolize death and decay.
Lines 1
The poem opens with the speaker addressing the rose.
The speaker tells the rose that it is sick.
Lines 2-4
The speaker describes an "invisible worm" that flies.
The worm can also fly when it's raining.
We don't know what this worm is doing in the poem or even what kind of worm it is. An invisible worm that can fly? Is it some kind
Lines 5-6
The speaker tells us more about the worm; it has found the rose's bed.
The status of this "bed" is ambiguous. It could be just a place where the rose sleeps that happens to be "crimson."
It could also be a "bed" of something, like a "bed of roses" (not unlikely since the poem is called "The Sick Rose") or something else that's red. This would make the rose a gardener of some kind.
Lines 7-8
The speaker tells us that the worm's "love" kills the rose.
It's strange that "love" is killing something here, since we usually associate love with life.
"Dark secret love" could mean three things. It could mean the worm's love, as in "My love for you will never die."
It could also refer to something that the worm loves, as in, "Hello, my love, I'm home."