The Pharisees, who previously had issues with Jesus, claimed that because of Jesus's self-witness, said witness is not true (as in reliable as according to Jewish law).
However, Jesus replied that even though he bears witness of Himself, His witness is still true. In contrast with John 5:31 (where Jesus was establishing his legitimacy based on the Mosaic Law's requirements of two witnesses), Jesus was referring to how even before the two witnesses, Jesus's claims are true (as in valid statements). Two different senses of "true" that Jesus used in the two instances. The two witnesses don't MAKE something true -- they DEMONSTRATE that something is true. As an analogy, Neptune was discovered by some scientists and accepted as a planet by the scientific community in the 1800s (Encyclopedia Britannica). However, that does not mean that before the 1800s, Neptune did not exist (or that from the discovery, Neptune popped into existence)! In a similar way, Jesus's words have been true the entire time -- the point of the two witnesses are to establish that for other people to know.
Jesus talked about how he knows where he came from and where he is going, unlike the Pharisees (8:14). We know that Jesus was sent from the Father, so it's likely that Jesus knows such things because of the Father.