Jesus did a lot of signs and works, from turning water to wine to lots of healings to raising Lazarus from the dead. Some people learned about Jesus and believed in Him. Other people, however, do not want to pay heed to Jesus, and some even want to capture Jesus.
John wrote that the unbelief of various people are a fulfillment of some parts of Isaiah. According to Merriam-Webster, "fulfill" can mean "measure up to" or "to put to effect". Thus, when John wrote that the unbelief fulfills Scripture, John refers to how attributes on the past are a greater descriptor of the present scenario (basically how history repeats itself). In some scenarios, the actions that happened are basically what Isaiah foretold would happen. Christopher Smith went into more detail on what fulfillment is in this article of the Good Question Blog.
The first fulfillment is in Isaiah 53. Isaiah 53 is a direct prophecy about someone now known as Jesus. Isaiah wrote that people would reject the message (God knows people of Isaiah's day with hearts hardened), and sure enough, that happened (as well as the rejection of Jesus).
The second fulfillment is from Isaiah 6. Isaiah 6 talks about Isaiah's commission and one of his earlier message he proclaimed to the Israelites. Take note that scholars like E.W. Bullinger note that some active verbs employ a common Hebrew figure of speech to indicate permissive action (letting something happen rather than direct action), so the section is explaining how God lets people blind themselves and harden themselves. In other words, the people reject Jesus themselves rather than God taking over their mind. The blinding and hardening accurately described the people who rejected Isaiah's message, and sure enough, the description fits the people who reject Jesus at the time of John 12.
Isaiah wrote these when he saw the glory of Jesus (especially since Isaiah 53 is a direct prophecy on Jesus himself).