In "Dule et Decorum Est," "Haunting flares" tells us the flares give off a: -deathly odor -spectral flavor -white light -sensual scent -none of these
Whenever you have a question like this that asks you to define the impact of a particular group of words, it is really important to go back to the text and read those words in context before proceeding to try and answer the question. In this example, it if important to consider the tone of the section where these words occur and what the poet is trying to achieve. Note the first four lines of the poem:
Bent double, like old beggars under sacks,
Knock-kneed, coughing like hags, we cursed through sludge,
Till on the haunting flares we turned our backs
And toward our distant rest began to trudge.
The overall effect is to present us with a hideous image of dehumanised soldiers marching around in a desolate, godforsaken landscape. Therefore, considering the bleak tone that is created, it is fair to say that the "haunting flares" add a spectral flavour to this poem.