Asking for help: what does this response from USCIS mean?

Called on June 2. never received an email response, but got it in the mail today:

“… we have to perform additional review on this case and this has caused a longer processing time. Please allow extra time for an Officer to review your application…. Your application remains awaiting review by an Officer and you will be notified as soon as a decision is made.”

Not really sure what it means though. Is the case still under "background check" or else? Can I call them again? Many thanks for any insights!!