In a word - everything! We release our ideas of ownership, our notions of control, and the delusion that anything in this life is permanent. Let go, let go, let go, and then let go of letting go.
Why do this? Happiness comes from cooperating with the design of life rather than fighting it.
Let go; cooperate with the design of life; be happy. So simple.
To God belongs the east and the west; wherever you go there will be the presence of God. God is Omnipresent, Omniscient. (Quran 2;115)
Such is God your Cherisher and Sustainer, there is no god except He, the Creator of all things. You shall worship Him alone. He is in control of all things. (Quran 6:102)
Know that this worldly life is no more than play and games, and boasting among you, and hoarding of money and children. It is like abundant rain that produces plants and pleases the disbelievers. But then the plants turn into useless hay, and are blown away by the wind. In the Hereafter there is either severe retribution, or forgiveness from God and approval. This worldly life is no more than a temporary illusion. (Quran 57:20)
Peace on you,