I think this verse is very interesting. Many people seem to think that the only reason Jesus came was to die on the cross for the propitiation/expiation of our sins. But Jesus says here that He has completed the work the Father gave Him to do; and this was BEFORE He was crucified. So what does He mean by that?
Well, I think (though I could be wrong) that He is speaking of a couple things involving His personal ministry. Jesus had spent His life telling people about God, especially in the last 3 years. Even more specifically, He had spent the last 3 years pouring His life into a group of 12 men, 3 of which He spent even more time with. He ate meals with them, He taught them, they went everywhere together. Jesus is the ultimate model of discipleship. He designed it. Discipleship is God's plan to advance the Kingdom. Be discipled, and in turn, disciple others.

John 17:4