In the previous five chapters, Jesus told the disciples about a big call, some big challenges, and some big promises. Then Jesus prayed for the disciples as well as all believers. But now comes the big time for Jesus.
Jesus and the disciples went into a garden. Eventually, he encountered Judas (AKA the one who ran off in John 13) with a bunch of troops and officers from the Pharisees (AKA the ones who opposed Jesus). At that time, Jesus "[knew] all things that would come upon Him" (18:4). Because Jesus knew the hearts of man, Jesus knew that the cross was inevitable. The "all things" is referring to the overall hardships that Jesus would face on the cross (though it's heavily implied to be much more than that).
Thus, Jesus asked the troops and officers who they were seeking, and in response, Jesus declared, "I am He." (18:4-5) Essentially, Jesus declared himself to the authorities (since they would ultimately put Jesus up on a cross). However, after saying these things, the authorities "drew back and fell on the ground". (18:6) They were likely surprised that Jesus would turn himself in (since most targets tend to hide from authorities rather than turn themselves in).