What is meant by “whatever goes into the mouth passes into the stomach and is expelled?”
This is a quote from Jesus, found in Matthew 15:17. The Pharisees had asked Jesus why His disciples were not ritually washing their hands before a meal. This was a Jewish tradition, but it was never commanded by God in his Word. So Jesus tells them that God’s Word is much more important than human traditions.
The purity of our hearts
Then He continues to teach that in the eyes of God the purity of our heart is the important thing. The Pharisees were very worried about how to eat and what to eat. To be ritually pure was an important part of their religion. Jesus tells them: don’t worry so much about your food. Not what goes into your mouth is important, but what comes out of it.
The unclean within us
The Pharisees are offended. But Jesus explains it to His disciples: You eat your food, you go to the toilet, and it’s gone. (That is the meaning of the verse you are asking about.) So it does not have the power to make you impure. But what comes out of your mouth shows what is in your heart. And what is in the heart of man? Jesus says: “evil thoughts, murder, adultery, sexual immorality, theft, false witness, slander” (Matthew 15:19).
These are the important issues and are the things that make us unclean. These are the things that need to be cleansed from our lives. That is only possible when we put our faith in Jesus. He gives us new life. He breaks the power of sin in our lives.
Get a good introduction about the Bible in What is the Bible?