The grade of a murmur just equals how easy it is to hear - ie grade 1 not easily detectable with a steth, up to grade 6 which you can hear with the naked ear close to the chest and can usually feel with your fingers on the chest.
A murmur is the sound caused by abnormal blood flow, which can be caused by a number of different problems, including faulty valves and a “hole in the heart” which you sometimes hear about in humans. They can also be normal variants, but you need to have the puppy properly checked before you know that for sure.
The grade of the murmur does not necessarily correlate to the seriousness of the problem. Generally the louder the murmur the more serious the problem but this is not always the case, it all depends on what is causing the murmur.
The only way to know exactly what the problem is and how serious it is is to get an echocardiogram, which is an ultrasound of the heart. You need to decide if you are happy to have this done, and then you need to decide what to do with the results – ie, if it is a serious problem will you have puppy pts? Are you prepared to have a dog on medication?
Hopefully your vet may have been able to give you some guidance on this when the murmur was found, and may have had some indication then of what the problem might be. I would either go back and talk to them about it or have the puppy seen by a vet you feel you are able to ask more questions of.
Good luck and I hope it all works out well for you.
