After Jesus was arrested, Jesus got interrogated by the high priest. The high priest asked about Jesus's teaching (possibly a specific one, but plausibly the whole) as well as His disciples (like a court case prosecutor). But Jesus provided an interesting response.
Jesus proclaimed that he "spoke openly to the world" and "taught in the synagogue" and "in secret [...] [had] said nothing" (18:20). The Greek word describing teaching frequency (Strong 3842) has been translated "ever" or "always", indicating that his teachings (especially ones that matter in the trial) are publicly widespread by now. Even in times like where Jesus uses parables, his explanation is available for any seeker who sticks around (Pharisees just didn't want it). Also, as noted in Matthew 10:27, Jesus intends that all explanations to come out publicly (rather than remaining with the disciples). Not to mention that people can choose to follow and learn from Jesus. Long story short, the lack of secret teachings means that those who want to know Jesus's teachings have the opportunity to do so. Thus, Jesus told the high priest to ask people who have heard Jesus's words (18:21).
Additionally, with His response, Jesus held to the legal process of interrogation, like the need to look for witnesses to establish a case as well as the role of personal confession in the Jewish legal system (as noted in Deuteronomy 19:15 and even the Sandhedrin). Jesus likely knew that the interrogation would go beyond what was legally required, so Jesus held to the legal requirements. This scene can be used as evidence for holding to legal rights in unjust prosecutions.