Meaning:
“人” means “a person,” and “从众” means “to follow the crowd,” which refers to “doing the same thing that most other people are doing, without really thinking about it for yourself.” Hence the term means “to follow the crowd.” As the three Chinese characters consist of one, two and three “人” respectively, people started to use them together for a humorous effect. Nowadays the term is mostly used by netizens to refer to the “huge crowds at tourist destinations during holidays.”
Example:
A: 你国庆假期去哪儿玩了？
nǐ guóqìng jiàqī qùnǎrér wán le ？
Did you travel to anywhere during the National Day holiday?
B: 我宅在家里。我想去的城市和景点一到假期都是“人从众”，所以我还是别的时间再去吧。
wǒ zhái zài jiālǐ 。 wǒ xiǎngqùde chénɡshì hé jǐngdiǎn yí dào jiàqī dōushì “ rén cóng zhòng ”， suǒyǐ wǒ háishi biéde shíjiān zài qù bā 。
I stayed at home. I do have some planned trips, but those cities and scenic spots are packed with tourists during holidays. I’d better go another time.