"Our Father who art in heaven."
The Lord's Prayer is the third chief part of the Catechism. It follows the Ten Commandments and the Creed. This progression is logical, or theological. The Second Commandment instructs us to pray ("pray, praise, and give thanks"), and now we are given the prayer. And the Creed (Father, Son, Holy Spirit) helps us to understand the Lord's Prayer, a prayer that is addressed to the Father, taught to us by the Son, and prayed with the help of the Holy Spirit.
The Lord's Prayer is not the most words, just the right ones - and the ones that are just right. In English the Lord's Prayer has a perfect seventy words. It's made up of seven parts called petitions (seven is the perfect number), together with an introduction and a conclusion.
The Introduction is "Our Father who art in heaven."
"What does this mean? With these words God tenderly invites us to believe that He is our true Father and that we are His true children, so that with all boldness and confidence we may ask Him as dear children ask their dear father."
So reads the Small Catechism. What a wonderful, warm explanation!
But, if I may, what else does this mean?
Answer: With these words God also invites us to believe that, since He is our true Father, we are true brothers and sisters, so that with all humility and joy we may love one another as He has loved us.
And if you put it all together, you can see the shape of the cross. Luther's "What does this mean?" expresses the vertical part of the cross, and my "What else does this mean?" expresses the horizontal.
And all of this is contained in the words "Our Father who art in heaven."
Pray them, and the whole prayer, slowly and from your heart!