"Everything is in the hands of heaven except the fear of heaven"
Rabbi Chanina (Berakhot 33b)

When Rabbi Chanina mentions the "fear of heaven" - he means "the fear of God". And by "fear" he does not mean fear as most people see it. This is one of those moments when Hebrew does not translate into English precisely. The term often translated as "fear" really means "reverence", "respect", and "honor". There is a big difference between fear and honor.  

Fear of Heaven

Being frightened or scared is involuntary. It could be the most disturbing, unsettling, and even traumatic experience. Respect and reverence, however, require our decision and intention. We always have a choice whether to respect and to honor someone. But fear is a reaction, not a decision. They are fundamentally different.

Rabbi Chanina says that everything is in God's hands. Almighty is absolutely capable of all things. But in one specific matter, God restricts his limitless abilities and does so on purpose. He does not force us to do his will but allows us to make a choice to honor him voluntarily. Because honor not given freely is not really honor. Coerced reverence is not true respect. That is why the Rabbi explains, that the fear of heaven, the human choice to honor God, is truly in our hands.