Tawakkul is reliance on God, the entrusting of outcomes to God rather than to one's own contrivance. "Whoever relies on God, He is sufficient for him" (65:3); "and when you have decided, then rely on God" (3:159), a verse whose sequence, decision first and reliance after, became central to the argument that followed.
That argument is over what tawakkul asks of effort. An early ascetic reading, pressed furthest in ninth-century Khurasani circles associated with Shaqiq al-Balkhi and among some of the stricter renunciants, treated the deliberate seeking of means, storing food, taking medicine, contracting for work, as a defect in trust, and produced practices such as travelling without provision. It had the merit of taking the verses at maximum strength.
The majority position, argued by al-Muhasibi, al-Ghazali and the jurists generally, held that tawakkul is a condition of the heart entirely compatible with the use of means, since God appointed the means and commanded work. Its proof text is the report of the man who asked whether to leave his camel untied and trust in God, and was told to tie it and trust in God. Al-Ghazali's treatment in the Ihya sorts believers into degrees, allowing that the highest may dispense with means while denying that this is required of anyone, and warning against imitating a state one has not reached.
The dispute has not gone away; it recurs whenever fatalism and medicine are discussed, and the classical materials are cited on both sides in modern debates over whether seeking treatment or taking precautions is compatible with trust. The mainstream juristic answer has consistently been that it is, and that neglecting means where they exist is not reliance but negligence.