What is session token? I can't get the session token using monday.get("sessiontoken") method in monday-sdk

Whenever I am trying to call the following code, it returns an error as follows

monday.get('sessionToken')
.then((token) => { 
  // send token to backend
  // On backend, call jwt.verify(token.data, 'MY_CLIENT_SECRET') 
}

It returns the following error -

Token: method: 'get', args: {…}, requestId: '7gj78ik', 
clientId: undefined,
version: '0.1.6'}
args: params: undefined
type: "sessionToken"
[[Prototype]]: Object
clientId: undefined
method: "get"
requestId: "7gj78ik"
version: "0.1.6"

why ‘clientID’ is undefined ? Also, what to do with this session token ? does it have any expiration time? can I use ‘session-token’ instead of ‘access-token’ received from OAuth?