Arabic for "If God wills it."
Muslims believe that nothing in this world can happen unless Allah allows it to happen. It is therefore recommended for one to say this phrase when they mention something that they wish to perform, in hopes that Allah will allow their plan to take place.
It is inappropriate to use this phrase for acts that are against Islamic jurisprudence.
Good Examples:
Insha'Allah, I will make hajj this year.
I will repay the debt within a week. Insha'Allah.
Bad Examples:
We'll go out an steal a car tomorrow, Insha'Allah.