Tawhīd is the Absolute Oneness and Unity of God. It is the most important and most fundamental aspect of Islam. Without Tawhīd, none of the uṣūl al-dīn, or Islam, would make sense. Tawhīd denies the existence of any other god or deity except for the One God who created the world.  

It also holds that God has no partners in anything He does. Although He may have servants that perform tasks in the world by His will, this does not mean that they are His partners in any way as He is in no need of them. He is perfectly capable of managing the world without them.  

Tawhīd is not just an abstract theological understanding of the nature of God, it has a direct bearing on how we lead our lives and how we are supposed to see the world around us. Tawhīd not only implies that we must serve and submit our hearts to Him only (that is, not money, our desires etc.) but it also means that only Allah controls and sustains the universe. Anything else is but a mere illusion. To give independent power to anything else in this regard is called shirk in Islam, that is, polytheism or idolatry. 

As a result, we shouldn't be upset when things do not go our way. For in the end, little is under our control. Everything belongs to Him, and He controls the results of all events that take place in our world.