Let's not get too complicated here - this simply means that JavaScript is a Turing Complete language. All Turing Complete languages can be used to create any computer program in theory at least.
The JavaScript engine itself is a program just like any other engine, compiler, interpreter, operating system, etc...
Thus, the JavaScript engine could be written in JavaScript itself.
I want to stop everyone here and mention a major point of stupidity in how us developers converse with one another - we often say things like "Python is written in C." Not only is that really not true, but it makes us sound like 4 year olds... The language itself is a syntax which is written by humans to express ideas to the interpreter or engine.
Python is not written in C. The Python interpreter is written in C. Likewise, JavaScript wouldn't be written in JavaScript, the JavaScript Engine/interpreter could be implemented in JavaScript.
However, in actual practice, a base JavaScript engine wouldn't generally be written in the same language because an engine would first need to be created for the language to interpret itself and thus that original engine must be written in another language.
This sound super cool but it's just hype because any language can be written in itself as long as it's Turing Complete, not just JavaScript.
In fact, as I've mentioned elsewhere here on HashNode, a man named Christopher Domas at Battele Institute showed that you can write any program at all in only mov instructions on the CPU, so therefore you could even write JavaScript by doing absolutely nothing else except assigning variables to other variables...
Now, that's a trip. :)