What does it mean when the codes say emissions are lower then recommended?

I don't know the code. I was just told the cat has lower then recommended emissions

No, a happy catalytic converter is present or you would be over threshold on your emissions. The car expects to burn "X" amount of fuel and air in a proper ration to one another. When the engine is burning too clean or dirty, the efficiency of the engine has been altered due to component failure. Finding out what has caused this issue will require you to have the engine professionally scanned during operation to determine what parameters are not being met. This will provide a more direct route to finding the issue through an understanding of individual component and system function. If you scan with a handheld scanner and find what codes are present, sometimes those codes can point in a good direction without the need for professional diagnosis. Sometimes.