What does Luke 13:18 mean?

"Therefore" indicates that Jesus tells these parables because of the prior passage: Jesus healing a woman on the Sabbath and thereby splitting the religious leaders from the people. The leaders are shamed and the people rejoice (Luke 13:10–17). Luke may have a grander view in mind. The kingdom of God has come (Luke 9:2). Healing the woman is a sign of the kingdom's power to restore health and defeat Satan (Luke 9:2).

But the Jews of Jesus' time don't understand that the inauguration of the kingdom of God begins a slow process of developing a people of God. This is necessary before they enter His more physically manifested kingdom on earth. Only when the Holy Spirit's work to build the church is complete will Jesus establish God's literal, physical kingdom at His second coming (Acts 1:6–7).

The healing of a single woman is a precursor to the day when all Jesus' followers will receive glorified bodies. The rejoicing of the people over the work of God to rescue her from bondage to Satan will see its culmination when Jesus defeats Satan completely (Revelation 20:7–10). Add to that Jesus' call to repentance from sins and reconciliation with God (Luke 13:1–9) as well as the people's acceptance of Jesus' power and authority as their Lord, Savior, King, and Priest. All these aspects of God's kingdom will take a long time to come to completion.

The slow growth of the mustard tree and the arduous work of the baker are metaphors for the gradual but saturating and inevitable work of the gospel.