Meaning:

“禁止” means “to forbid,” and “套娃” refers to “matryoshka dolls,” also known as “Russian nesting dolls.” This toy consists of a set of four to eight hollow wooden dolls of graduated size nesting inside each other. Chinese netizens use the saying to warn others against repeating or copying existing contents online, based on the implication that the Russian dolls nesting inside each other are repetitions of a smaller size.

Example:

A: 我发给你的视频看了吗？

Wǒ fā gěi nǐ de shìpín kàn le ma？

Did you watch the video I sent?

B: 挺好笑的。不过这个视频是模仿别人的，底下评论都是“禁止套娃”。

Tǐng hǎoxiào de。Búguò zhè ge shìpín shì mófǎng biérén de，dǐxià pínglùn dōu shì “jìnzhǐ tàowá”。

It’s hilarious. In fact, It’s a copycat, and you can see many comments at the bottom saying “Copycats are forbidden.”