a murmur is simply a defective heart valve that does not close properly leaking blood back into the ventricles, usually the left one. some cases murmurs
are acquired thoughout life because there is a systemic problem that causing it like hypertension
. depending how bad is leaking you should avoid extreme sports, keep blood pressure low with medication if needed.
about the dentist
visit, he wants to avoid you developing an infection somehow. I don't think it relates to the murmur, since it's a cardiovascular problem.