Hukam, from the Arabic hukm, command, is in Sikh teaching the divine order or will by which everything happens. It is among the central terms of Guru Nanak's theology and appears almost immediately in the Japji Sahib, in the question of how one becomes truthful and how the wall of falsehood is broken, answered: by walking in the hukam, which is written within you.
The concept does two things at once. It describes: everything that exists and occurs does so within an order that is God's, and Japji's second stanza states that all forms, all greatness and lowness, all pleasure and pain come by hukam, and that nothing whatever stands outside it. And it prescribes: the fitting human response is hukam manana, accepting and living in accordance with that order. This acceptance is presented as the direct alternative to haumai, self-centredness, the person who insists on their own will is living against the grain of what is, and suffers accordingly.
The tradition is careful that this is not fatalism, and the point is worth making because the doctrine is easily mistaken for one. Sikh teaching insists throughout on effort, honest labour, moral responsibility and action on behalf of others, and the scripture's statement that as one acts so one receives is as characteristic as anything it says about hukam. What acceptance excludes is not effort but the demand that reality conform to the ego's preference, and Sikh commentators treat the two as complementary rather than competing.
A related and distinct usage should be separated out. A hukamnama was historically a written order issued by a Guru, and later by the Akal Takht, to the community, and such directives have continued to be issued and to carry real authority. In everyday practice today the word names something else as well: the passage read at random from the Guru Granth Sahib when a gurdwara opens each morning and at the conclusion of ceremonies, taken as the guidance of the day for the congregation. The same word thus names the cosmic order and the particular direction given to a community on a particular morning, and Sikh commentary connects the two deliberately rather than treating the usage as a coincidence.