Will I need surgery for an Incisional Hernia?

I have been told by my Cardiac Nurse that I have an Incisional hernia, a lump in the middle of my chest at the bottom of my sternum (after CABG). A lump pushes out when I cough etc. she just said if it bothers you come back. I thought a hernia could be serious. Is it ok to ignore it. What has caused it?