According to John, Jesus is telling the people that He knows exactly who He is and where He is going, even if they don't understand it. This means that Jesus is sure about His mission and purpose, even if others are confused.
John 8:14: Jesus answered and said unto them, Though I bear record of myself, [yet] my record is true: for I know whence I came, and whither I go; but ye cannot tell whence I come, and whither I go.
In this verse, Jesus states that if He bears witness of Himself, His witness is not true, which contrasts with John 8:14 where He claims His witness is true even if He bears it of Himself.
John 5:31: If I bear witness of myself, my witness is not true.