An allegory has a literal and metaphorical meaning.

Before Dante can enter into each ring of hell he first passes through the Gate of Hell. There he finds people who were opportunists or wouldn’t chose right from wrong morally and just stayed neutral. The reason they are right outside hell is because they’re the closest to heaven but still part of hell, because they never chose. They continually chase a blank banner while getting stung by wasps and having their blood and tears consumed by worms.

Although this is literal it is also metaphorical because the reason they are chasing the banner is because they never made the choice between right and wrong. The banner represents the choice they never made and their meaningless stay on earth, hence the blankness. Also the wasps keep them moving and worms respresent the payback for the people who the opportunists wronged to better themselves, because the worms are taking their blood and tears.