My IgG level is 147 iu/ml. What does this mean?

Quite low. Normal levels in most labs start at around 600. It depends on whether that is your total igg or a subclass that is 147. If it is your total, you would probably have frequent infections, but you haven't told us this. If it has only been checked once, we would start by rechecking it again, to be sure the result is accurate and yours. Then more tests. You need a specialist in immunology.