This might be referring to two things, possibly.
The idea that the language could be used to write a parser that could parse itself.
That a newer version of JavaScript could be transpiled to an older version of JavaScript, again, using JavaScript. (Like Babel.)
I haven't heard anyone say this, but I would guess if they said it, they are referring to #1.
Edit: To answer ahead of time - the purpose of this is less about actually doing it, and more about explaining what the language is capable of doing. JavaScript is capable of a lot of stuff. There's probably a more useful descriptor of "language that can implement lexing", but that's beyond the scope of my answer, and apparently beyond the scope of my quick Googling as well. Someone else on here will know what it is.