The expression “In Shaa’ Allah” (if Allah wills) means making any future act a Muslim wants to do contingent upon the will of Allah.

Saying "In Shaa' Allah" is part of the etiquette of Islam. The Muslim is instructed to say "In Shaa' Allah" whenever he expresses his wish to carry out a future action.

And never say of anything, “I will definitely do this tomorrow,” without adding, “if Allah so wills!” (...)

Quran - 18:23-24

(interpretation of the meaning)

When the person says it in connection to his future actions, these actions may or may not take place; however, it is more hoped that they would take place if he says "in Shaa' Allah."

The story of Prophet Sulaymaan (Solomon) indicates this meaning.

The Prophet ﷺ (peace be upon him) said, “Solomon (the son of) David said, 'Tonight I will sleep with (my) seventy wives, each of whom will conceive a child who will be a knight fighting for Allah's Cause.' His companion said, 'If Allah wills.' But Solomon did not say so; therefore none of those women got pregnant except one who gave birth to a half child.” The Prophet ﷺ (peace be upon him) further said, “If the Prophet Solomon had said it (i.e. 'If Allah wills') he would have begotten children who would have fought in Allah's Cause.”

Sahih al-Bukhari, 3424

This is proof that when someone says "In Shaa' Allah," it is more likely that his wish and need will be fulfilled; however, this is not necessarily the case all the time. If Allah wills it, it shall be fulfilled. It should be noted that deprivation could be a blessing in disguise.

(...) Perhaps you dislike something which is good for you and like something which is bad for you. Allah knows and you do not know.

Quran - 2:216

(interpretation of the meaning)

Source: islamweb.net · islamweb.net