To be able to understand, you need to know the meaning of the Internet Slang in the sentence. There’s two Internet slangs:
安利 (Anli) – Anli is the Chinese name for Amway. In internet slang, the meaning is to share or to recommend. It also mean hitting the “share”.
入坑 (rukeng) – literal meaning is enter hole. In colloquial usage, this means fallen into a trap. In internet slang, it means becoming addicted to or obsessed with something or someone. Normally, this refers to being addicted to a tv series or web novel series.
他被安利入坑 basically means he fell for it due to recommendation. I’m assuming that he started to like something like a song or TV series after reading the review or being recommended by someone he knows or after reading the shared content on the social media page of someone he knows.
Other examples of Internet slang :
666 – pronounced as liu-liu-liu; means awesome
双击 shuangji – literal meaning double hit; in Internet slang, this means hitting the “Like”.
路人 luren – not a fan, casual follower; the literal meaning of the Chinese word is bystander
圈粉 quanfen – becoming a fan, started to follow, the literal meaning is round powder but the powder (粉 fen) is actually an abbreviation of 粉丝 (fensi), the transliteration of the word Fans. However, the original meaning of 粉丝 is glass noodle.
私生饭 sishengfan – the obssssive fan of an idol who treat the idol as personal property and often engaged in behavior detrimental to the idol’s well being, like stalking the idol. The phrase is a transliteration of the Korean phrase Sasaeng, which mean the same thing. The literal meaning of the Chinese phrase is illegitimate rice.