I cloned a git repository and saw strange texts in the files. HEAD <<< and >>>>>

For example, I see the code

<<<<<<< HEAD
    public function login()
    {
        if($this->_identity===null) {
            $this->_identity=new UserIdentity($this->username,$this->password);
            $this->_identity->authenticate();
        }
        if($this->_identity->errorCode===UserIdentity::ERROR_NONE) {
=======
    /**
     * Logs in the user using the given username and password in the model.
     * @return boolean whether login is successful
     */
    public function login()
    {
        if($this->_identity===null)
        {
            $this->_identity=new UserIdentity($this->username,$this->password);
            $this->_identity->authenticate();
        }
        if($this->_identity->errorCode===UserIdentity::ERROR_NONE)
        {
>>>>>>> 6f9cd42a2926dccc612900c31769a977e71ed261
            $duration=$this->rememberMe ? 3600*24*30 : 0; // 30 days
            Yii::app()->user->login($this->_identity,$duration);
            return true;
        }
<<<<<<< HEAD
        else return false;
=======
        else
        return false;
>>>>>>> 6f9cd42a2926dccc612900c31769a977e71ed261
    }
}

and I didn’t know what that meant. Can someone explain?