Congrats on your new kitten! 🙂
My guess is that your vet might have said "sinus arrhythmia" if you truly heard the word "sinus" in there. I think sinus arrythmias are much more common in dogs--or at least, much easier to hear! Come to think of it, they might not ever occur in a cat...sorry for the musing. Anyways, if you think you might have heard the word "systolic," then that's the most likely scenario.
The term "systolic" simply refers to the phase of the cardiac cycle (systole) during which the murmur can be heard. This gives an indication of which valve(s) are likely to be affected, depending on the exact sound and location of the murmur. (If a valve is supposed to be closed during systole and there's a leak, you hear a murmur.)
However, both puppies and kittens can have "innocent" murmurs that disappear as they get older. Typically, these murmurs disappear by the age of six months. Life expectancy is completely normal for these guys!
Cats are also weird and can have so-called "physiologic" murmurs in stressful situations (such as going to the vet). These murmurs are transient and not reliably reproducible. My stupid cat cost me $500 because I worked up a "new" murmur--only for him to have a completely normal echocardiogram. Ah, well--would rather he be healthy!
Unless it's really loud or your kitten is showing signs of cardiovascular problems (collapse, weakness, exercise intolerance, lethargy), then your vet will likely just watch it and hopefully the murmur will disappear.
Cool, huh?