What is the definition and exact rules of Rabbanut or Rabbinate Kosher?

ב"ה Shalom, This question is too broad spectrum to give you an answer. To go in to all the issues of Kashrut, would be like writing the Shulchan Aruch. There are halachot that apply to meat, fish, eggs, fruits and vegetables, milk products, Chalav Yisrael, ,insect infestation, wine, bishul yisrael, gelatin ,and more. Although, I am not the spokesperson for the Rabbanut, I can say without a doubt the guidelines of the Rabbanut are the Shulchan Aruch. But around the Shulchan Aruch, especially when it comes to food many people like to be more stringent. The Rabbanut does not enforce all stringencies, but that does not make the food not kosher. It means that those who are strict in a particular regard, would refrain from eating such an item. I will give one example, chocolate or ice cream made with milk powder, which is not Chalav Yisrael. The Rabbanut will give it a Hechsher, since many Rabbis allowed using milk powder which is not Chalav Yisrael, but the Rabbanut will also state on the label, "Kosher for those who eat milk powder which is not Chalav yisrael." (In Hebrew) Therefore, those who eat only Chalav Yisrael will refrain from eating such chocolate, and those who are not strict about it are eating kosher , following the Rabbis who allow it. This is a bit similar to some kosher agencies in the US, who give a hechsher to chocolate or ice cream, which is not Chalav Yisrael, but will specify the ice creams which do have Chalav Yisrael. Therefore, to be practical, if you are planning to go to restaurant and there is a particular issue that you want to inquire about, the phone number of the mashgiach is available, and you can ask what he does in regard to the particular item in question and then you can consult your Rabbi . All the best