What do people use Symbol functions in Javascript for?

I recently came across some code that was using syntax that I wasn't too familiar with. At first, it appeared to be a function that was part of an object, but I can't seem to access it. I've found information here on it, but I'm still not sure why someone would use this over a standard function signature. This is an example of the code, and what I'm trying to do with it.

const secretFunction = Symbol('secretFunction');

class SecretController {
  constructor(){
    ...
  }
  [secretFunction](){
    console.log("I AM THE SECRET!");
  }
}

So, what I am trying to do is test this code with a tool called jest. I'm doing so like:

let secretController = new SecretController();
let secretFunction = Symbol('secretFunction');

describe("TEST", function(){
  it('What is this?', function(){
    secretController[secretFunction](); // <-- doesn't work
  })
})

I've tried this a few other ways, but I get the same type of result. That is:

TypeError: secretController[secretFunction] is not a function

What is the purpose of writing a function this way? How do you use functions like this? How do you test them?