today's other readings are Numbers 21: 4-9 and Psalm 102
An easy to overlook aspect of John's gospel is Jesus' frequent use of "I am". In our English speaking eyes, it makes sense that Jesus uses this term as it is the first person form of the verb "to be".
But in the case above, Jesus isn't referring just to His earthly presence, but He is referring to the name God used in referring to Himself "I AM" in His appearance to Moses in the burning bush in Exodus 3. By referring to Himself as "I AM", Jesus infuriates His opponents who accuse Him of heresy.
But there is another important phrase in today's gospel that requires comment: “When you have lifted up the Son of Man”, which links Him to two important Old Testament stories.
One is today’s first reading from Numbers 21:4-9, an account of the attack of serpents in the desert on the people when they complain about lack of food and drink. Moses makes a bronze image of the serpent and lifts it up so that anyone who looks at it can be healed. By linking His reference to being lifted up with the saving image of the pole Moses creates, we gain insight into the purpose of the saving cross of Jesus Christ.
Jesus also calls Himself the “Son of Man”. This title is taken from the Book of Daniel (7:14-14), which speaks of a mysterious figure, both human and divine, who comes on the clouds on the day of judgment. This title is one of the few that Jesus accepts, and is doing so fully embraces His humanity. But for St. John Jesus also is the perfect representative of humanity before God, the "Son of Man."