The phrase insha'Allah means “God willing” or “if God wills it” in Arabic. Devout Muslims say "insha'Allah" whenever they make a statement about a plan to do something, in a way of requesting God to bless the activity. The phrase also acknowledges submission to God, with the speaker putting him or herself into God's hands, and accepting the fact that God sometimes works in inscrutable ways.
In the Qu'ran, Muslims are told that they should never say they will do a particular thing in the future without adding "insha'Allah" to the statement. Some Christians are surprised to learn that a similar sentiment also appears in the Bible, in the Epistle of James, which says that people should remember that they never know what tomorrow will bring, so the will of God should always be acknowledged when making plans.
In a classic parable about why Muslims should say "insha'Allah" before doing something, a Muslim encounters a friend on the way to market, and says that he intends to buy something there, but does not add "insha'Allah" because he is confident that the deed will be done. When he reaches the market and goes to complete the transaction, he discovers that his purse has been stolen, rendering him unable to make the purchase, and his wife chides him, saying that he should have said "insha'Allah."
As a general rule, one says "insha'Allah" when making any statement about a plan, such as “I am going to buy a car tomorrow, insha'Allah.” If the phrase is accidentally omitted, some people believe that it can be added later, as soon as the omission is realized, but "insha'Allah" cannot be said after the fact, because God's will has obviously already been done.
A related phrase is masha'Allah, which means “God has willed it.” Both phrases exemplify the idea that devout Muslims submit themselves to the will of God as part of their religious faith. A Muslim who follows the Pillars of Islam and behaves in a righteous way also hopes to incur the blessings of God by saying "insha'Allah," but he or she also accepts that God may have other plans for the outcome of a pledge to do something.
Visitors to the Middle East often hear "insha'Allah" used as a euphemism for “we'll see,” which can be a source of frustration for some people. It can help to remember that most people are too polite to say that something simply will not happen, so adding "insha'Allah" to a statement can express the idea that something is, in a sense, up to God, whether it be catching a train at the right time or completing a deal to sell a house.