The second argument of Jesus against the Pharisees was a proverb he told the crowd. Later, in a private setting, Jesus explained it to his disciples (7:17–23). It concerns moral distinctions about eating (rules about what is right or wrong to eat). Eating affects the system that processes food, but moral issues involve the heart. The heart is a metaphor for where a person makes moral decisions.
How a person eats (with ceremonially clean or unclean hands) or what a person eats (ceremonially clean or unclean food) only affects the system that processes food. So it does not impact the moral issues of the human soul or inner being. To help his later audience understand the meaning of this teaching, Mark adds this comment Thus all foods are clean.
In the early church, Peter and Paul helped make this meaning clear (see Acts 10:1–11:18; Romans 14:14; 1 Corinthians 8:8; Galatians 2:11–21).