I don't like rolling my own cigarettes, so I always buy pre-rolled ones.

pre-rolled 是由前缀 pre-（表示"预先"）和动词 roll（卷）的过去分词 rolled 组合而成的形容词。pre-rolled 字面意思是"预先卷好的"，在这里指"已经卷好的香烟"，即"现成的香烟"。