When one is in prolonged confinement and cut off from happenings, one can be most anxious about one's unfinished businesses. John was in prison (Mt 11:2). He wanted to know if Jesus was "the stronger one" as he had predicted, the one on whom he saw the Holy Spirit come down and rest during baptism, or should he keep expecting and waiting for another one?
Jesus' ministry was gaining traction in Galilee. Jesus asked John's messengers to be his witnesses by recalling what they had seen and heard, the miracles which he had worked, miracles which had fulfilled the prophecy of Isaiah. Then they should report back to John what they had seen and heard.
Many people still ask John's question in various ways: Is Jesus really God? Was he truly the promised redeemer? As witnesses and messengers, we implore the Holy Spirit to point us to God's working through Jesus, so that we may see and hear and so help others to experience that Jesus is truly the One sent by God.
May God be blessed.