The gemara (Shavuos 20b) classifies vows into three categories:
1) An oath on something which occurred in the past, e.g. I swear I ate that piece of bread. Swearing this type of oath falsely is a violation of shavu'as shav.
2) An oath to do something in the future, e.g. I swear I will eat that piece of bread for lunch. Swearing this type of oath falsely is a shavua's sheker.
3) Taking a neder and violating it, e.g. This piece of bread is forbidden to me like a korban. Violating a neder is an issur of bal yacheil. (Tosfos adds that bal yacheil applies to the previous cases as well in addition to the particular issurim of shavu'ah.)
Rashi writes a chiddush that gives the gemara more meaning than just creating categories. A person must receive hasra'ah, a warning not to do what he is doing, before receiving punishment for violating an issur. It's not enough (says Rashi) to tell a person in general that what he is doing in wrong, but rather hasra'ah must mention the specific prohibition being violated. If a person is violating shavu'as shav, it does not good to warn him that he is violating a shavua'as sheker, or vica versa.
The Rambam makes no mention of the requirement to mention the specific lav being violated as part of the warning of hasra'ah. I asked my son what he thought the logic behind Rashi's opinion is, but the truth is I don't fully grasp it myself. Clearly hasra'ah must be more than a warning that something wrong is being done, or a simple "That is prohibited" would suffice. But why require mentioning the specific issur being violated?
On the same topic, my son quoted what sounds to me like a very strange sevara that his Rebbe heard from R' Zelig Epstein. Why is it that ain mazhirin min hadin, that the Torah warning not to do an issur must be spelled out in a pasuk and not derived from a kal v'chomer? R' Z.E. explained that were the issur derived from a kal v'chomer, there would be no way to formulate a proper hasra'ah on a specific lav. It sounds to me like R' Z.E. understood b'pashtus that hasra'ah is not a general statement, but must point to a specific lav -- I don't know why you would take that as a given when it is a chiddush of Rashi that needs explanation. Secondly, I think a simpler reason why ain mazhirin min hadin (assuming you even need to say a sevara instead of just accepting it as a gezeiras hakasuv, see Yevamos 24) is because there is always the chance that the kal v'chomer you use to derive a new issur is incorrect. I'm a bit baffled by the question and answer.