OATUH 2.0 Without SDK - Authentication URL

I am learning how to utilize OAUTH 2.0 in my React application. In the docs I see:

let accessToken = await axios.post(
  authenticationUrl,
  querystring.stringify({
    grant_type: 'authorization_code',
    code: '[CODE]',
    client_id: '[CLIENT_ID]',
    client_secret: '[CLIENT_SECRET]'
  })
)
.then(response => response.data.access_token)

 What does authenticationUrl refer to? Is this the same as authorizationUrl as stated in step 2 or is this the url that is returned with the code after I am redirected? Thanks