openerp.ia_document_chatter = function(session, mail) {        openerp.ia_document_chatter is a class like void main. and it shoud state name of the module.
    var _t = session.web._t, This is variable
       _lt = session.web._lt; This is variable
       
       openerp.mail_followers.Followers.include({ mail_followers.Followers is class in parent js. and we use .include to make it inheritance. We need to use openerp. same like openerp.tools for example.
       
        start: function() { This is overrided method avaible in mail_followers.Followers parent class.
        $("<br/><div><h4>Document ID</h4><input class='document_id' type='text' name='document_id' readonly='readonly'/></div>").insertAfter(self.$('div.oe_follower_list')) Here we have added text field document id directly from js.
        this._super(); #call super
        model_name = this.view.model #get current model
        record_id = this.view.datarecord.id #get record id.
        this.$('.document_id').val("/**" + model_name + "&" + record_id + "**/") # fill the value in document_id text field.
        }
       })
       
}