Hukam is order, command, the way things are. The Japji states early that everything is within it and nothing is outside it, and asks how one becomes true, answering that it is by walking in the hukam as it is written.
It is not fatalism. The Sikh tradition's history is not the history of a fatalist community. The distinction drawn is between accepting what is and being passive about what one does, and the entire martial tradition sits inside that distinction.
Bhana manna, accepting the will, is the phrase used at a death and at a disaster. It is also what a Punjabi family says to itself after a crop fails.
The daily hukamnama, the passage read at random in the gurdwara each morning, applies the word in its other sense: an instruction, taken as addressed to whoever is listening.
A tradition that fought the Mughal empire for a century and teaches acceptance of the divine order at the same time is not being inconsistent, and understanding why is most of understanding it.
Hukam is the order, the command, the way things are, and the Japji sets it as the thing that cannot be described and that everything is inside.
The line that answers the question of how to become true says: walk in the hukam. That is the whole instruction and the rest of the composition is an unfolding of it.
The word is also the daily practice. Taking a hukamnama, opening the volume at random and reading the hymn found there, is how a Sikh household begins the day.