In Matthew 6, Jesus contrasts those who pray in public to gain the admiration of people around them with instructions for His disciples to pray in the inner-most area of their homes—as far away from the public as possible.
What is a prayer closet?
He said, "And when you pray, you must not be like the hypocrites. For they love to stand and pray in the synagogues and at the street corners, that they may be seen by others. Truly, I say to you, they have received their reward. But when you pray, go into your room and shut the door and pray to your Father who is in secret. And your Father who sees in secret will reward you" (Matthew 6:5–6).
The word "room" here is the Greek tameion meaning an inner storage chamber, or secret room. Jesus was communicating that prayer is about God, not about the watching world. Rather than pray boisterously in the most public way possible so as to garner respect or appear righteous before others, we are to pray with our hearts set on God. Prayer is communing with Him, not performing for others.
Some Christians have taken Jesus' admonition literally and set aside a room, or even a closet, dedicated to prayer. Most Christians who want to follow this instruction simply identify a specific, quiet place in which to pray regularly. For some it is a certain chair or a place at a desk or the kitchen table. For others, it is their car during their commute to work each day or a place on the porch or in the woods. Famously, for John Wesley's mother Susanna, it was under her apron which she upturned over her head for solitude even with several children vying for her attention.
Having a quiet, regular place to pray can help us to focus and concentrate on prayer. Many also use this their "prayer closet" for Bible study. God speaks to us through His Word; studying the Bible goes hand-in-hand with praying. Jesus Himself often retreated to quiet places to pray, many times on a hillside.
Jesus' instruction is less about where you pray and more about the mistake of drawing attention to our piety or self-righteousness as we pray. He preaches about praying, giving, and serving as a response to God's goodness to us, rather than a way to show others our own goodness. The concept of a "prayer closet" is simply to remember that prayer is about fellowship with God. A "prayer closet" can be anywhere we are free to experience uninterrupted intimacy with God.
Is silent prayer biblical?
What does the Bible say about public prayer? Is it okay to pray in public?
What is the importance of daily prayer in the life of a Christian?
Why should we study the Bible?
Why should I spend time alone with God?
Truth about Prayer