What does John 20:30 mean?

In the prior passage, Jesus was mildly scolding Thomas and the other disciples for being resistant (John 20:29). They pushed back against believing Christ had been resurrected until they saw Him in person (John 20:19–23). They'd ignored eyewitness accounts and prophecy to do so (John 2:19–22; 20:17–18; Luke 24:10–11; 23–24). They were especially blessed to have been given so much evidence, since those who came to faith in Christ after would not have access to those experiences.

This verse alludes to those unmentioned details. John's gospel was written well after the other three, which is one reason he skips over certain details to add his own. Each writer has a purpose, and each passage is meant to convey certain ideas. John's choice of what to include, or exclude, is based on that purpose. Here, he notes there are many things of which he could have written but did not. Some of that is a matter of limited space (John 21:25). The rest is about maintaining the book's purpose.

In the next verse, John will explicitly explain the ultimate purpose of his choices: to focus on Jesus as the Messiah, and God, so those who read will accept Him as Savior (John 20:31).