In the previous verses, Jesus has just corrected a false view of giving, which was (and still is) one of the pillars of faith in God. Now He addresses another pillar of this faith: prayer. Jesus' focus here is still on, as He stated back in verse 1, not "practicing your righteousness before other people in order to be seen by them..."
It is assumed in the life of a Christian that he/she is a person of prayer. This is why Jesus begins this section by saying, "And WHEN you pray..." (emphasis added). Albert Mohler says it like this, "In Scripture it is unthinkable that a true disciple of the Lord Jesus Christ would not pray. Being a disciple of Jesus means following after him, walking as he walked, and doing what he taught. This means praying as he prayed. An active prayer life is assumed."
As Jesus did with giving, He clearly states that our praying should not happen in order to be seen by others, but rather to be heard by our heavenly Father. The reward of hypocrites who want to be seen for their "piety" is the praise of man. Now, this may not be the temptation for many of us. Many of us shy away from public prayer. And Jesus is not condemning all public prayer (there are many New Testament examples of times of corporate prayer). But it does beg us to ask the question, why do I pray? And what is my motivation in prayer?
Hopefully our answer to that is to please the Father, and not man, and to learn to trust Him more. Much of our prayer life is learning to trust God more with our cares, concerns, needs, etc. May we all learn to trust God more as we pray.
Not only are we not to pray like the hypocritical Pharisees, Jesus calls His followers to not pray like the pagan idol-worshipers of the day, who would recite and chant the same words or phrases over and over again. They thought that if they prayed long enough and hard enough, the gods of the day would answer them. But Jesus makes it very clear that the Father knows what we need before we ask Him. God does not need our many words. He simply wants our honest, faithful requests to be entrusted to His care.
As we will see next week, Jesus perfectly models this kind of prayer life for us. And we see this perfect model of prayer for us throughout the Gospel accounts. Charles Quarles sums it up well: "Although Jesus was devoted to prayer, His prayers were not memorized recitations given at the whim of the clock. His prayers were intensely personal, often spontaneous, and an expression of His deep communion with His Father."