What is Kosher?
In many of the articles you’ll find online discussing what is kosher, ultimately the discussion seems always to touch upon, or more than touch upon, the issue of Meat and Milk.
What is Kosher Food?
The issue of meat and milk is really very simple to explain. The Torah, the Jewish Bible, prohibits the eating of meat and milk together. Indeed, after someone ate meat, he is prohibited under Jewish religious law from eating dairy products for six hours (in most communities) as a Rabbinic separation mechanism.
Additionally, what is Kosher food implies to prohibit even to cook meat and milk together, and to derive benefit from the resulting product if they're cooked together, like selling it or feeding it to your animal.
Besides the prohibition to cook milk and meat together, it is just as forbidden to cook meat products in a pot that was used for milk, or vise versa, milk products in a pot that was used for meat. This is because the Torah reckons also with the taste of the previously cooked food, which comes out into the contents when you cook the second thing, and therefore this is also included in cooking and eating meat and milk together.
In an actual case where meat and milk were cooked together, or meat in a milky pot (a pot that was previously used for milk) one should consult with a competent Orthodox Rabbi, because whether the food became forbidden or not is dependant on several relevant details, amongst them being the ratio of the different foods involved, how long ago the pot was used for dairy, and whether sharp foods were involved, like onions.
That is why in Kosher homes they have two sets of dishes and pots, one for meat and one for milk, and the same is true in an eating establishment like a restaurant. Usually a Kosher restaurant will serve only meaty (and neutral) foods or milky and neutral foods. In a fast action environment like a restaurant it is hard enough to keep control of the Kashrus (the being Kosher) without the issue of meat and milk mixups.