When explaining the loud sound, Jesus foreshadowed His death and resurrection and proclaimed that he would later draw everyone to himself. But some people had some questions. They thought that the Christ remains forever, so they have trouble when they heard that the Son of Man must be lifted up, let alone knowing who the Son of Man is. Though it's worth noting that after the resurrection, the Christ reigns and remains.
In response to the question, Jesus told those people that they need to rely on the light while it is still available. Kinda like doing afternoon activities like sports before the daytime ends and night begins. Just like the prior analogies of light (as introduced in John 1), Jesus is the light and ultimate guidance. He also calls them to "believe in the light" so that they can "become sons of light" (12:36). In other words, Jesus wants them to learn about Him and His reliability so they can count on Jesus while He is still there in person.
In fact, Jesus talked about walking with the light "lest darkness overtake [them]" (12:35). Darkness is the opposite of light, and we can't really see much in a dark room. Thus, the darkness is referencing the people shutting Jesus (AKA the light) out of the life. Once they insist on shutting themselves from Jesus, they can't really do jack or even have effective direction (as Jesus is the light and ultimate guide).
Today, as Jesus is still the ultimate guide, it's better to learn from Him than to shut ourselves from Him.