standing grain: a cereal crop grown to full height and still rooted in the field, not yet cut.
Hebrew farming had a word for each stage of a grain crop, and this one names the middle of it: the plants grown to full height, still rooted where they were sown, heads ripe, waiting on the sickle. It is a snapshot of one moment. Once the reaper cuts the crop it stops being this word and becomes sheaves, and then a stack (גָּדִישׁ).
That timing is what makes the word useful to the writers who reach for it. The laws use it to fix a moment of risk and a moment of mercy: a fire loose in a field destroys whatever is still standing, and a hungry passerby may pull heads from a neighbor's crop by hand but may not put a sickle to it. Samson picks the same moment to burn the Philistine fields, when a whole year of work is dry, upright, and one torch away from ashes.
The prophets use it for what a harvest ought to be and is not. Isaiah shrinks a nation down to the few ears a reaper's arm can gather (שִׁבֹּלֶת), and Hosea says the crop is standing there with nothing in its heads, so there will be no flour. In one taunt against Assyria the word turns into a measure of growth instead of a crop: grass on a roof shrivels before it ever gets that tall.