I presume they said a 1/6 murmur, which is how we classify them. A 1 is very soft, very difficult to hear and seldom indicative of heart
You have probably had it a long time, but no one heard it (again, very soft). If they said don't worry, don't.
A murmur is a sound, not a disease. They can mean a problem, but more are not. Ask your doctor directly, is it innocent?