It's a noble title that translates as above a duke but below a grand duke or Royal prince. Royal princes being what most people think of princes (I.e sons of the King). The difference in Russia to the Uk at least is that all members of the family are entitled to use the honorific as it is the family rather than the individual that has princely status. So if your father is some high ranking general and prince let's say, all sons and daughters are born princes or princesses.