Three short lines, and one surprising comparison. The speaker names a feeling, jealousy, and then pictures it as "a hundred hips on the rose." Rose hips are the small round fruits that form on a rose after the flower has finished blooming. The title, "Rosa canina," is the Latin name for the dog rose, a wild rose that produces them.
The image works by contrast. A rose usually stands for love or beauty, but here the poem points to what comes after the petals: a crowd of hard little fruits, often red. Jealousy seems to grow the same way, as something that ripens out of love and keeps multiplying. "A hundred" suggests it isn't a single pang but a whole cluster of them.
The mood is quiet and a little wry. The speaker doesn't shout or complain, just observes, almost like noticing a bush by the road. That calm tone makes the feeling seem more stubborn and real.