Verse 9 tells us what this parable is about. It is about people who are confident in their own righteousness and look down on others. It is a parable about true righteousness.
The Pharisee is a good man who does the right thing and observes all the religious practices. The tax collector represents sinners. He is the kind of person who others regard as bad people.
But he goes home justified before God. God has given him his approval. Why? Because he asked for mercy (18.13). The Pharisee does not gain God’s approval. Why? Because he boasting about himself. He should have humbled himself and asked for mercy as well.
Jesus says this applies to everyone. Anyone wants to boast to God about themselves will be rejected. Those who ask for mercy will be approved as righteous by God.
Dale