Insha'allah is "If God wills" or "God willing" are common phrases Muslims use regularly and quite frequently in their daily conversations.
If you ever happen to have a conversation with a Muslim you will always find him ending most of his sentences about anything that he will do in the future with the phrase "If God wills" (in sha' Allah in Arabic), be it something he will do in the next coming minutes or after ten years.
Muslims believe that nothing in the heavens or the earth happens unless God wants it to happen. Part of believing in the Oneness of God and true submission to Him is believing in His full control and power over everything He created.
A firm believer in God knows that God gave him limited powers, he can't assure to himself that he will live to the following day, let alone guaranteeing taking his second breath. So he always attributes any will or power to God, for He is the one Who gives it to him.
When the pagans asked Prophet Muhammad (peace be upon him) to tell them about "the Sleepers of the Cave", the story of "Zul-Qarnain" and the soul, he said tomorrow I will tell you the answers of your questions, without saying "Insha'allah" (If God wills).
Prophet Muhammad (peace be upon him) waited for a revelation from God with the answers for these questions for fifteen days. Then God revealed to him the following verses reminding him that no one should say tomorrow I will do so and so without saying If God wills,
[And do not say of anything: Surely I will do it tomorrow, Unless Allah pleases; and remember your Lord when you forget and say: Maybe my Lord will guide me to a nearer course to the right than this.] (Al-Kahf 18:23-24)
This, of course, is not an open invitation to be passive, break promises, and lay back, saying, "I have no power whatsoever over my life."
Reliance on God for help and sustenance, and admitting His power should not be interpreted as passivity of action in one's life, for God ordered us to work hard and at the same time seek His guidance and help and rely on His wisdom for deciding what is better for us.
As we don’t know the future, and the Unseen belongs to God only. We should trust in His divine providence.
God says in the Quran what means,
[Say: I do not control any benefit or harm for my own soul except as Allah please; and had I known the Unseen I would have had much of good and no evil would have touched me; I am nothing but a warner and the giver of good news to a people who believe.] (Al-A`raf7:188)
Believing in God's power over everything is a kind of relief for every Muslim, for he knows that God only decrees what's good for him. When faced with ordeals and calamities, Muslims believe that God will always guide them to a way out.
They truly mean it when they say " Tomorrow will be better, if God wills" for they know that their Lord only does what's best for them, and from Him comes all the bounties and all the Good.