There are three “heavens” in the Bible.
#1 is the sky
#2 is outer space.
#3 is the place where God’s presence is manifested like nowhere else in creation. This is the place modern people think of when they use the term “heaven.” 2 Corinthians 12:2 speaks about this 3rd heaven.
Jesus uses the word heaven twice when he teaches the disciples to pray.
“Our Father in heaven, hallowed be your name. Your kingdom come, your will be done, on earth as it is in heaven. Give us this day our daily bread, and forgive us our debts, as we also have forgiven our debtors. And lead us not into temptation, but deliver us from evil.” Matthew 6:9-13 (ESV)
The first use of the word “heaven” is in fact plural (heavens). This reminds us that God occupies all of his creation--all three heavens.
The second use of the word “heaven” is singular. Here heaven is referring to the place where God’s presence is manifested. In this place God’s will is followed completely (as opposed to earth).
Jesus was saying that when we pray, we should petition that God’s intentions will be carried out fully on earth just like they are in heaven.