Thanks so much both

I agree with Lily on her interpretation.

It means someone sucks but he pretends he is very good at.It's like "不懂装懂“（He who knows nothing but pretends to know) But the Chinese people sometimes is very modest, if you are a really talented person told you this idiom does not mean he sucks, it's just a humble way of saying.

太好了，，，，非常感谢！

I found the answer on the internet Simplified 滥竽充数 (làn yú chōng shù): to acquire a position or play the part without necessary skills. Cannot be used freely to strangers. They might get angry. Ex: 他是个滥竽充数演员 He's a terrible actor. Can be used to describe someone is not good at their job.