Because the disciples (and to an extent all believers) are "not of the world", Jesus prays for the Father to help the disciples not succumb to sinful values favored by the world. Additionally, Jesus asks the Father to "[s]anctify them by [the Father's] truth" (17:17). In particular, Jesus is referring to the Father's word, which is truth.
According to Merriam-Webster, "sanctify" has multiple meanings -- "to set apart for a sacred purpose" or "to free from sin". It's likely that Jesus had both meanings in mind. A few verses ago, Jesus talked about how the disciples were "not of the world" -- especially since the disciples chose Jesus while the world rejected Jesus. Thus, God essentially set apart the disciples since the disciples chose Jesus. Additionally, the Father's truth (especially with the call to love one another and to accept Jesus) is specifically designed to help the disciples (and by extension, us) be less susceptible to sin since truth provides a firm foundation. In fact, those in heaven are completely free from sin.
Additionally, Jesus talked about how the disciples are sent into the world -- even though the disciples are "not of the world". This is because with the sanctification, the disciples are essentially distinguished from the world -- not to mention that they can unleash the light for other people in the world to learn about Jesus, His character, and His gift.