The capturers sent Jesus to Pilate for Pilate to deal with Jesus so the capturers don't have to. Now Pilate asks Jesus about some matters to see if Jesus should really get killed.
The first question that Pilate asked is whether or not Jesus was the king of the Jews. Regarding the question, Jesus first asked if the source is from Pilate or from others (18:34). Jesus responded such a way because different people can ask the same question with different motives. In particular, Pilate talked about how he is trying to learn about Jesus and if Jesus was a king.
In that case, Jesus responded that his kingdom is "not of the world" (18:36). If the kingdom is of the world, according to Jesus, Jesus's servants would fight. However, at that time, only Peter tried defending Jesus, and even then, Jesus rebuked Peter. We've seen Jesus use the "not of the world" analogy multiple times in his talk with the disciples (as seen in John 15 and John 17), and what Jesus meant when he said "not of the world" is that Jesus focused on God and God's kingdom rather than physical land and power.
With the "not of the world" kingdom likely in mind, Pilate then asked Jesus if he is a king. In that case, according to Jesus, yes. Jesus talked about how he was born to "bear witness to the truth" (18:37). We've seen Jesus talk about his link to the Father and how Jesus would provide the way to everlasting life multiple times. Showing others the truth is part of Jesus's big goals on Earth. In fact, with the sayings, Jesus even told such truth to Pilate himself!
From Pilate's own investigation, Pilate had difficulty knowing what Jesus meant by "truth", but Pilate found no reason to find fault in Jesus. So Pilate told the Judeans that Pilate does not find fault in Jesus. Pilate's findings mean that Jesus was pronounced innocent.