We use may, could or might with practically no difference in meaning to say that it is possible that something will happen or is happening.
They may be arriving tomorrow.
He might be away on holiday.
It could be true.
The negative of may is may not. The negative of might is might not. They both mean that it is possible that something will not happen or is not happening. Remember that we DO NOT use could not to express probability.
He may not pass his driving test.
They might not come to visit us tomorrow.