coding an application in angularjs, i have an error of sintaxis?

in this example the code works

but when i try to pass this on my server i have an error

ReferenceError: _ is not defined

this is the js code

// Declare app level module which depends on filters, and services
var app = angular.module('myApp', []);

function RegisterCtrl($scope, $location) 
{

  $scope.steps = [
    'Step 1: Team Info',
    'Step 2: Campaign Info',
    'Step 3: Campaign Media'
  ];
  $scope.selection = $scope.steps[0];

  $scope.getCurrentStepIndex = function()
  {
    // Get the index of the current step given selection
    return _.indexOf($scope.steps, $scope.selection);
  };

  // Go to a defined step index
  $scope.goToStep = function(index) 
  {
    if ( !_.isUndefined($scope.steps[index]) )
    {
      $scope.selection = $scope.steps[index];
    }
  };

  $scope.hasNextStep = function(){
    var stepIndex = $scope.getCurrentStepIndex();
    var nextStep = stepIndex + 1;
    // Return true if there is a next step, false if not
    return !_.isUndefined($scope.steps[nextStep]);
  };

  $scope.hasPreviousStep = function(){
    var stepIndex = $scope.getCurrentStepIndex();
    var previousStep = stepIndex - 1;
    // Return true if there is a next step, false if not
    return !_.isUndefined($scope.steps[previousStep]);
  };

  $scope.incrementStep = function() {
    if ( $scope.hasNextStep() )
    {
      var stepIndex = $scope.getCurrentStepIndex();
      var nextStep = stepIndex + 1;
      $scope.selection = $scope.steps[nextStep];
    }
  };

  $scope.decrementStep = function() {
    if ( $scope.hasPreviousStep() )
    {
      var stepIndex = $scope.getCurrentStepIndex();
      var previousStep = stepIndex - 1;
      $scope.selection = $scope.steps[previousStep];
    }
  };
}

what is the function of

$scope.goToStep = function(index) 
  {
    if ( !_.isUndefined($scope.steps[index]) )
    {
      $scope.selection = $scope.steps[index];
    }
  };

why is the error???