What does this letter mean?Please help me with that

I received letter from uscis a coulpe of days ago after I called them.

The status of this service request is:

Based on your request we researched the status of this case.We are actively processing this case.However,we have to perform additional review on this case and this has caused a longer processing time.If you do not receive a decision or other notice of action from us within 60 days of this letter,please call customer service at the nummber provided below.

any explanation will be wlcome.
Thanks!