What does John 8:26 mean?

Here again, Jesus claims that the message He brings is that of God the Father. This echoes statements He has made earlier in His ministry (John 6:38–40), as well as in this same conversation (John 8:16). In the prior verse, Jesus' critics delivered a question about His identity. More than likely, this was meant in a critical, rebuking way: "who do you think you are?" All the same, Jesus has been consistent about His testimony. He has pointed to the Scriptures (John 5:39–40), miracles (John 5:36), the witness of others (John 5:32–33), and His own unique knowledge (John 8:14–16). All of these give Him the right to claim to be the light of the world (John 8:12), and to point out that those who reject Him will suffer spiritual death for their sins (John 8:21). In short, Jesus is the Promised One, and God incarnate.

All of this, sadly, is lost on the men attacking Jesus. As the next verse indicates, they simply do not understand. At the same time, Scripture makes it clear that a person's intent comes before their ability to understand. The evidence is there, and the truth is there, but those who do not want to understand cannot understand (John 7:17). Jesus is not going to waste time giving proofs or evidence to those who have no interest whatsoever in truth (Matthew 7:6).