5 years ago I was told I had a heart murmur and then a year or so later was told it had gone. What is a murmur? And how did it just go away?

Depends on your age. About 40% of children have heart murmurs, most of which are normal sounds (functional or "innocent" murmurs) that come and go. Murmurs due to some types of heart malformations, such as ventricular septal defects, may also resolve over time. In adults, murmurs that go away may also be functional, especially during pregnancy, or due to anemia or other illnesses.
Turbulence. Turbulence across a heart valve causes a noise heard through the stethoscope; this is called a murmur. Depending upon the situation, a murmur can be physiologic (that is, within the range of normal). We all can have situations (which may be temporary) in which there is more turbulence (high, noisy flow) over the heart valve.