“What does low hemoglobin mean after surgery?”

Means you lost blood during surgery

You probably lost some blood during surgery. Your body will replace this providing you have enough iron stores.

Low hemoglobin (hgb) after surgery means there was likely some blood loss during or after your surgery. There are other causes like dilution if you were dehydrated prior to surgery, so what I am saying is "in general" of course. Hgb is in the blood and carries oxygen to your tissues. If your levels were normal prior to surgery then having low levels means there was either blood loss or dilution from the IV fluid. The operative note should clearly state the "estimated blood loss" during the surgery or just ask your surgeon. All surgery is serious, but during some surgeries, a large amount of blood loss is expected and other surgeries a transfusion is likely to replace expected blood loss after levels of Hgb go down below 7 or so. If there was blood loss and you refuse a transfusion (due to religious reasons, i.e., JW), then an acute loss from normal (15) down to less than 5 can result in serious serious complications. Death is much more common when Hgb is <5 in this setting. To rebuild your own Hgb, you will need iron as that is the limiting factor commonly. If your Hgb is 10 or so, then that is common and it should slowly return to your pre-surgical levels in a couple of months. Problems from blood loss come immediately during or after the blood loss so the fact you're writing this question tells me you're doing pretty good. 
Good luck with your continued recovery. Recovery should take 1-2 months before you feel great. It's important not to smoke, maintain ideal fitness, and decrease your stress to make your recovery the best it can be.

It’s pretty common after surgery. It’s due to blood loss or dilution from lots of fluid!