The passage you are referring to means "Do not boil a kid in its mothers milk." That means you shouldn't cook a baby goat in the milk of a goat. There were dietary laws that state:
- the prohibition against cooking a mixture of milk and meat
- the prohibition against eating a cooked mixture of milk and meat
- the prohibition against deriving any benefit from a cooked mixture of milk and meat
One possible reason is that some of the nations participated in that practice in their religion.