You can read a full explanation of the Jewish Dietary Laws here, but in a nutshell keeping kosher means:
- All meat must come from animals who have cloven hoofs and chew their cud, plus the animal must have been slaughtered following the kashrut guidelines
- You can only eat fish with fins & scales
- You don't mix meat in it's mother's milk (ie you don't mix meat and dairy in the same meal)
- Foods are broken down into 3 categories - dairy, meat, and parve (from the Yiddish word "pareve" which means neutral. ex. fish, fruits, vegetables, grains)
A kosher home will also have separate dishes, pots, pans, utensils, etc for meat and dairy. Parve dishes can be made and served using either meat or dairy. A home of a very observant Jew might have separate sinks, dishwashers, ovens, etc for meat and dairy.