"Stubble" and "chaff" are the useless parts of the food grain plant. During harvest they are dry and so burn easily. (See: [[rc://en/ta/man/jit/figs-simile]])
Isaiah speaks of these people as if they were a dying plant. Alternate translation: "they will die like a plant whose roots have rotted and whose blossom has dried up and blown away in the wind" (See: [[rc://en/ta/man/jit/figs-metaphor]])