Jesus is praying to God the Father. So Jesus is interceding for His disciples, just as the Bible says He does for all who know Him. To 'sanctify' means to cleanse or purify or set apart. Those who follow Christ for salvation are eternally forgiven. But practically, every Christian still struggles with sin. God desires Christians to become more and more loving, patient, kind, holy...to be transformed from the inside out to become more like Jesus Himself, set apart as 'salt and light' in the world. That is why Jesus is praying this request for His disciples and in the broader context for believers of all generations. Jesus makes it clear later in John 17 He is praying for future disciples who would believe on His Name.
One of the ways this process takes place is through God's Word, which is not just truthful, but truth itself. Jesus is also called the Word, and interestingly, Jesus is called the Way...the Truth...the Life. Unless a Christian hears, reads, studies, memorizes and meditates on God's Word (the Bible), he/she cannot grow in this sanctifying process.