The Kingdom of Heaven is one of those misunderstood ideas as it is conflated with the Kingdom of God, that they are the same thing, but Jesus uses them is a specific way which are different from each other.
Matthew is the only Gospel writer to include this term "Kingdom of Heaven", even though he still uses the term "Kingdom of God". The others Gospel writers use the phrase “the kingdom of God” only. So, what is “the kingdom of heaven?”
The Kingdom of Heaven is the physical reality of the Kingdom of God, being confined to those who are a part of the Kingdom of God, i.e., those who trust in the Lord Jesus Christ.
It covers a number of different dispensations of time, meaning it is not limited to the church age, or millennial kingdom (although some would suggest that this is what is meant by the "Kingdom of Heaven"), but the scriptures seem to indicate, the Kingdom of God in the context of the world in which believers live is the Kingdom of Heaven
Matthew 7:21 "If anyone says to Me, 'Lord, Lord,' is not automatically going to enter the kingdom of heaven, but the one who does the will of My Father in heaven."
Matt 11:12 Since John the Baptist until now, the kingdom of heaven suffers, and they will take it forcefully with violence.
Matthew 13:24-25 Another parable He put forth to them, saying: "The kingdom of heaven is like a man who sowed good seed in his field; but while men slept, his enemy came and sowed tares among the wheat and went his way.
Matthew 20:1 "For the kingdom of heaven is like a landowner who sought labourers to hire for his vineyard early in the morning."