Meaning:
“路” is short for “路人,” which means “passer-by,” “转” means to “convert into,” and “粉” is short for “粉丝,” which is the transliteration of the English term “fans.” This saying, first used in the discussion of celebrities, refers to the process of “becoming a fan” of a certain celebrity. In Chinese, the term “passer-by” is a metaphor for “strangers having no interest in a celebrity.” The phrase can also be used in a more general context where people find themselves becoming a fan of a movie, TV show or a product.
Example:
A: 你说要买一双跑步鞋，买了没？
Nǐ shuō yào mǎi yīshuāng pǎobùxié，mǎileméi？
You said you want to buy a pair of running shoes. Have you bought them?
B: 买了国货精品李宁，才穿了一天，果断路转粉了，设计简洁，脚感舒适，而且比耐克便宜好多。
Mǎi le guóhuò jīngpǐn lǐníng，cái chuān le yītiān，guǒduàn lùzhuǎnfěn le，shèjì jiǎnjié，jiǎogǎn shūshì，érqiě bǐ nàikè piányí hǎoduō。
I did. I bought a pair of Lining sneakers, a well-known Chinese brand, wore it for a day and have become a fan. The sneakers, with a simple design, are comfortable and cost much less than Nike.