We mentioned a person who is... He is also punished by stoning. The Mishnah says that a person would only be executed for this particular sin. If he did it on purpose with no witnesses and no warning, then he would be karet. Hashem would deal with him. He would be punished in the next world in a very severe manner, or he would die young. In this world, that's what karet is. That would be the punishment for this type of violation if he did it without witnesses or without a warning.
And what we're saying here, basically, is that if a person violated one of the 39 melachot (categories of prohibited labor) that we know traditionally are violations on a high level, if he did it with no witnesses, he would be karet. If he did it by mistake, he would have to bring korban chatat (sin offering). If he did one of these melachot in front of witnesses and with warnings, then the Bet Din (rabbinical court) would execute him.
But if a person violated another type of Shabbat law that wasn't part of the 39 melachot, even though it's from the Torah—for example, let's say the techum Shabbat, you're not allowed to walk more than 2,000 amot (cubits) from a person's town, which is a place where you can't walk—you're not allowed to walk more than 2,000 amot. According to some, this is de'oraita (Torah-level), but it's not one of the 39 melachot. For that, a person would violate lo ta'aseh (negative commandment), but he wouldn't get execution.
So what we mean over here, simply, is that when you're mechalel Shabbat (desecrating Shabbat), it's one of the 39 melachot that's being done on purpose with witnesses and proper warning.
Ha-mekalel aviv ve'imo, somebody who curses his mother, his father or mother; eino chayav ad sheyekalelam b'Shem, he wouldn't be liable to execution until he cursed them using the name of Hashem. If he cursed them with a substitute name of Hashem, he said, for example, ha-Rachum, merciful. He didn't use the name Hashem; he said “merciful,” which is alluding to Hashem. According to Rabbi Meir, he is still liable for execution, but the hachamim (the sages) say that he is not liable for execution.