What does Matthew 18:11 mean?

This verse is not in the earliest manuscripts of the book of Matthew, which translators use to develop modern English Bibles. That's why it is not included in many translations, including the English Standard Version (ESV) or New International Version (NIV). That these words were not originally part of Matthew' writing does not make them untrue. Most likely, these were added at some point to harmonize the account with Luke 19:10, which is included in all translations: "For the Son of Man came to seek and to save the lost."

The sentiment is correct, but modern translators don't believe Matthew wrote those specific words at this point in his own account of Jesus' life.