"Trim" is the ability to correct for deviations in flight controls by prepositioning the flight surfaces for a particular direction. For example, if the aircraft is pitching up a bit by default, you can trim down in order to counteract it.
"Out of trim" would mean that the Aircraft has either been improperly trimmed, or is operating outside of the range trim can be applied. (If the aircraft is pulling down hard, you can only trim up so much.) This can be due to a mechanical defect, improper weight and balance, or other issues. He may not be able to safely guarantee the aircraft is trimmed in such conditions.
There is a related question here that explains it in further detail: What is an out-of-trim condition and how is it detected?