Kosher dietary laws come from two sources in Judaism. The first source is the Scriptural source: the set of laws, rules, and protocol found in the Torah. These scriptures outline specific rules concerning forbidden and permitted animals, lawful slaughter, the correct use of utensils, laws for passover and fasting, and lawful sacrifice.
Kosher law is further developed in the second source, the talmudic tradition. The Talmudic tradition is the oral tradition of the rabbis passed down from geneartion to generation. Within the Talmudic tradition, kosher law has changed and evolved to address the concerns of modern living. For example, most foods have artificial preservatives and other lab-produced ingredients. These ingredients present special problems when considering Kosher dietary restrictions. Rabbis continue to expand, refine, and clarify the original dietary laws in order to address these problems.
Furthermore, a number of dietary laws related to sacrifice are only relevant for those living in Jerusalem, as they can only be enacted at the Second Temple. Thus, the rabbinical tradition also provides clarification concerning the way in which devout followers can maintain Kosher diets and traditions outside of the city of Jerusalem.
Eating Kosher means to eat correctly according to the laws in the Torah. There are many rules that need to be followed, for example you can't mix milk based foods with meat, because that is said to mix the milk of the mother with the meat. You have also to have a Rabbi saying that the food was kosher. For example, for meat, the animal needs to be killed without pain and all his blood should fall in the ground, as Judaism believes that the soul of the animal is in its blood. These rules come from thousands of years, and they keep being pretty literal today, as well as other rules, such as not lighting candles (or in our modern world, lights, computer, cellphones and cars), and many other rules from the Torah that are still literally followed by religious of Judaism. People who follow the Kosher way of life can't eat in non-kosher restaurants, or even eat in a house that is not known to be kosher, since residuals of forbidden meats (such as pork or some types of fish) can be present in pans or ovens.