Deuteronomy 14:9-10 meaning

Having dealt with which of the land animals the Israelites may or may not eat, Moses continued to deal with the matter of diet in regard to water creatures. He gave a general rule that these you may eat of all that are in water. Specifically, he told them that anything that has fins and scales you may eat. Fins are appendages (an external body part) used by fish to swim, maintain position, to move, steer, and stop. Scales are small rigid plates that grow out of the fish's skin.

By contrast, Moses explained to the Israelites that anything that does not have fins and scales you shall not eat (v. 10). This would include creatures such as shellfish, eels, and sharks since they did not have both fins and scales. So, the people were to regard these water creatures as unclean. In fact, Leviticus 11:10 states that such water animals (both freshwater and saltwater) were to be considered detestable to Israel. Therefore, they were not be eaten by God's covenant people.