The Rambam (Shabbos ch 29) paskens that it is forbidden to eat before kiddush during the day ("kiddusha rabbah") just like at night. The Ra'avad disagrees, arguing that since one can say kiddush over bread, and kiddusha rabbah is no more than a birchas hane'henin, there can be no issur of eating before kiddush because the bracha on the seudah IS kiddush. The simple approach to understanding the Rambam is that he holds one may not say kiddush over bread during the day. The seudah requires some way to demarcate it as a seudas shabbos beyond saying hamotzi. I am more concerned with the Ra'avad's objection. Why did the Ra'avad not think there can be an issur of eating before kiddush? Even if you assume you can say kiddush on bread, what if I wanted to eat an apple before kiddush - that is certainly not a seudah which qualifies as seudas shabbos, so according to the Ra'avad, how was I yotzei kiddush before eating?
It seems to me that the dispute between the Rambam and Ra'avad is how to understand the issur of eating before kiddush. According the the Rambam, it is an issur gavra independent of what one is saying kiddush on. According to the Ra'avad, it is a din in seudas shabbos, i.e. that seudas Shabbos must be preceded by kiddush. Therefore, there is no kashe from eating an apple - since by definition that is not a seudas Shabbos, there is not halacha (acc. to Ra'avad) which would prohibit eating it without kiddush.