In the Old Testament, the Lord revealed to Moses and the ancient Israelites that only certain foods were considered clean or, in other words, fit to be eaten. The distinction that the Israelites drew between clean and unclean foods had a great effect upon their religious and social life. Certain animals, birds, and fish were regarded as clean and acceptable to eat, while others were unclean and were forbidden (Lev. 11; Deut. 14:3–20). Some diseased persons were also considered unclean.
In a spiritual sense, to be clean is to be free from sin and sinful desires. In this sense the word is used to describe a person who is virtuous and has a pure heart (Ps. 24:4). God’s covenant people have always had special instructions to be clean (3 Ne. 20:41; D&C 38:42; 133:5).