To help us help you, could you please prepare a reproducible example (reprex) illustrating your issue? Please have a look at this guide, to see how to create one:
This looks like the result of an str() call on a list object. Without the code you use we can't help you.
As it was said above, you'll have in a rmarkdown document the result of any code you print in your chunk. Nothing more. So I would advice to check your code chunk to see what piece of code is printing this.