A thorn is a sharp spike that protrudes from the stem of a plant. Roses are famous for having the sweetest fragrance and the sharpest thorns.
Literal thorns, found on bushes and other plants, serve to protect plants from animals that might like to eat them. A "thorn in one's side" is an annoyance or problem: "My neighbors are a thorn in my side, with their constant loud music and crazy parties!" The source of this figurative definition is the Bible's "thorn in the flesh."