My doctor says I have a prominent sternum but didn't go to much into detail what it meant. Do you know what it means?
The sternum is the cartilage in the center of your chest. For most people, there is a dip in the chest where the sternum is. In a few people the center sticks out. Here is an example of a more extreme case, called pectus carinatum or pigeon chest, from Wikipedia:
The largest concern is that it can cause a person to tire more quickly because they don't take in air as efficiently. If you have a mild case, you probably never notice any difference.