If you're looking for the original biblical reference, there are a few that forbid boiling a kid in its mother's milk. I believe it's the origin of the prohibition of mixing milk and meat.
Those who keep kosher strictly separate both the preparation and consumption of milk and meat, eating milk products with only other milk products or with pareve foods, which are neither milk nor meat, and only eating meat products with other meat or pareve foods. Those keeping kosher also maintain separate plates and cookery, one set for milk and one for meat.
Mark