I can relate to that! Anyone who is really trying to do their best can understand what John said. The Lord always gives us big jobs to complete--like taking care of a family, taking advantage of an education, or just being a good friend. Often times we don't know what to do to do a good job. We're just mortals. We're imperfect. We don't know everything.
Yet the Lord still entrusts us with big jobs. After John baptized Jesus, John heard the voice of the Father bearing record that Jesus was His Son. He saw the Holy Ghost descend in the form of the dove, which represents peace.
Whenever we complete a big job that the Lord gave us, we too will know more surely that Jesus is the Son of God, and we will feel a peace in our hearts knowing that we did what He wanted. I know it happens that way because I've done it.